Where did the L in talk go?

Yes, it's regional. An interesting study of regional (US) variations in pronunciation, as well as terminology, was done at Harvard (the Harvard Dialect Survey) and the survey data were converted to a series of maps by Joshua Katz at NCSU:

http://www4.uwm.edu/FLL/linguistics/dialect/maps.html

There is an American Dialect Society, who discuss in great detail, for example, what has happened to the "aw" sound that supposedly distinguishes "hawk" from "hock".

In standard pronunciation the l is silent in "talk/walk" and similar words (see list above). This is a matter of simplification of pronunciation. After the long vowel /o:/ (this is not the correct phonetic sign) the consonant group lk is regularly simplified to /k/ as the clear pronunciation of /l+k/ would be cumbersome.

I pronounce the L in talk, walk, chalk, and other similarly spelt words. I believe it's a subtlety that is lost with familiarity. In fast speech, the pronunciation of the L is indistinguishable from saying the word without the L. This is why I believe most people believe that the L is silent. In slow speech, the L is noticeable.

I've noticed in Southern United States accents where the pace of the speech is slower and the contraction "y'all" is used a lot, you'll hear the L.

The people that I've met that pronounce the L like I do interestingly have something else in common with me, they have tried to shed their regional accent. I think people who become very conscious of how they enunciate words tend to work harder at the subtleties of pronunciation. Those that don't just settle for or acquire the regional accent.

Where did the L in talk go?

It's present in the spelling, but not in the pronunciation.
L-vocalisation is what caused walk, talk, yolk to have a silent L. It's a process whereby a lateral approximant [l] or [ɫ] is replaced by a vowel or a semi-vowel. The /l/ before a /k/ is often velarised, so I assume the [ɫ] changed the preceding vowel to a glide /w/ and then it became vocalised, but kept the spelling with an L.

From Practical Phonetics and Phonology: A resource book for students (Routledge English Language Introductions):

Dark l occurs before consonants and pause. The articulation is slightly velarised, with a concave upper surface, giving a back-central vowel [ʊ]-type resonance, e.g. still [stɪɫ], help [heɫp]. Dark l is often a syllable-bearer, when it will be of longer duration [ɫː], e.g. hospital /'hɒspɪtl̩/ ['hɒspɪtl̩ː]. Younger NRP speakers, especially those brought up in the area of London or the south-east, nowadays regularly have a vocalic dark l sounding rather like [ʊ], especially following central and back vowels, e.g. doll [dɒʊ], pearl [pɜːʊ]. This effect is termed l-vocalisation.

As per Wikipedia:

Historical diphthongization before /l/: Diphthongization occurred since Early Modern English in certain -al- and -ol- sequences before coronal or velar consonants, or at the end of a word or morpheme. In these sequences, /al/ became /awl/ and then /ɑul/, while /ɔl/ became /ɔwl/ and then /ɔul/. Both of these merged with existing diphthongs: /ɑu/ as in law and /ɔu/ as in throw.

At the end of a word or morpheme, this produced all, ball, call, fall, gall, hall, mall, small, squall, stall, pall, tall, thrall, wall, control, droll, extol, knoll, poll, roll, scroll, stroll, swollen, toll, and troll. The word shall did not follow this trend, and remains /ʃæl/ today.

Before coronal consonants, this produced Alderney, alter, bald, balderdash, false, falter, halt, malt, palsy, salt, Wald, Walter, bold, cold, fold, gold, hold, molten, mould/mold, old, shoulder (earlier sholder), smolder, told, and wold (in the sense of "tract of land"). As with shall, the word shalt did not follow this trend, and remains /ʃælt/ today.

Before /k/, this produced balk, caulk/calk, chalk, Dundalk, falcon, stalk, talk, walk, folk, Polk, and yolk.

Historical L-vocalization: In most circumstances, the changes stopped there. But in -alk and -olk words, the /l/ disappeared entirely in most accents (with the notable exception of Hiberno-English). This change caused /ɑulk/ to become /ɑuk/, and /ɔulk/ to become /ɔuk/. Even outside Ireland, some of these words have more than one pronunciation that retains the /l/ sound, especially in American English where spelling pronunciations caused partial or full reversal of L-vocalization in a handful of cases:

• caulk/calk can be /ˈkɔːlk/ or /ˈkɔːk/.
• falcon can be /ˈfælkən/, /ˈfɔːlkən/ or /ˈfɔːkən/.
• yolk can be /ˈjoʊlk/ or /ˈjoʊk/. yoke as /ˈjoʊk/ is only conditionally homophonous.

Modern L-vocalization: More extensive L-vocalization is a notable feature of certain dialects of English, including Cockney, Estuary English, New York English, New Zealand English, Pittsburgh and Philadelphia English, in which an /l/ sound occurring at the end of a word or before a consonant is pronounced as some sort of close back vocoid, e.g., [w], [o] or [ʊ]. The resulting sound may not always be rounded. The precise phonetic quality varies. It can be heard occasionally in the dialect of the English East Midlands, where words ending in -old can be pronounced /oʊd/.

[English-language vowel changes before historic /l/]

According to the Wikipedia article on Pronunciation of English ⟨a⟩:

Other pronunciations of the letter ⟨a⟩ in English have come about through:

• Rounding caused by a following dark L (which may no longer be sounded), to produce (in RP) the sound /ɔː/ in also, alter, ball, call, chalk, halt, talk, etc.. See English-language vowel changes before historic /l/.
• Rounding following /w/, resulting in the same two vowels as above, as in wash, what, quantity, water, warm. This change is typically blocked before a velar consonant, as in wag, quack and twang, and is also absent in swam (the irregular past tense of swim).

According to this paper on L-vocalisation:

In certain contexts, namely after present day /ɑ:/ and /ɔ:/ and before labials and velars, /l/ was vocalised in the 16th century. In almost all dialects of English today, therefore, a lateral consonant is absent in ‘calf’, ‘palm’, ‘talk’ and ‘stalk’, for example.