What does "OMG ponies!" mean?

According to Urban Dictionary:

an involuntary exclamation when confronted by extreme awesomeness or cuteness

"I logged on and and saw the pics of your new l33t rig and thought "omg ponies!"

l33t (from 1337) => leet => elite (gamespeak)

I could be mistaken, but I believe this exact phrase originated on Slashdot, in an April Fools' prank on April 1, 2006.

From the Wikipedia page on Slashdot:

In observance of April Fools' Day (April 1) in 2006, Slashdot temporarily changed its signature teal color theme to a warm palette of bubblegum pink and changed its masthead from the usual, "News for Nerds" motto to, "OMG!!! Ponies!!!" Editors joked that this was done to increase female readership.

The exact Slashdot article said this:

Our marketing department has done extensive research over the last 3 quarters and discovered that our audience is strangely disproportionately skewed towards males. Like, 98.3% males to be precise. To correct this oversight, we have decided to subtly tweak Slashdot's design and content to widen our appeal to these less active demographics. Don't worry! We'll still continue to serve our core audience, but we hope you'll work with us as we try to find a balance that will work for all.

There was a stream of articles on the site that all followed this theme of writing, using lots of caps and texting phrases like LOL, OMG, lots of exclamation points, and getting excited about things like My Little Pony, glitter, the O.C., and other things young girls would be interested in.

Here is the Slashdot site logo from that day:

The meaning of the joke is that this is how young girls write things. So, this phrase is used as a way to jokingly react like a 12-year old girl would.

While Slashdot April Fools' prank popularized the term, those pranksters probably took it from a Cute Overload post from a day before. The site Cute Overload was fantastically popular in its day.

This is as close as I have found to an authoritative definition of the phrase:

[13-year-old voice] LOL!!!!!!!! BBQ!!!! OMG, Sooooo kewl!!! CUTEST PONIE EVAR!!! AAAAAAAAAAAAAAAAAAAAAH! dies. dies again. ^_^ KBYE! [/13]