What vegetarian substitute for prosciutto could I use in Carbonara?

I've not tried it, but to my mind sun-dried tomatoes, thinly sliced, should work. How about bell pepper, grilled until the skin goes black to give a nice smoky flavour?

I'd recommend thinly sliced and seared shiitake mushrooms for an umami boost and appropriate texture. Really get a nice brown crust on them.

If you're after emulating a carbonara it's worth noting that it doesn't feature prosciutto anywhere between the ingredients, it uses bacon, and it's supposed to be soft (not crispy), fat and closer to small cubes (diced) than slices if you cut it yourself. The closest thing in texture you will get are most likely mushrooms, salting them and working towards taking away any aftertaste is up to you.

One vegan substitute would be some tempeh.

The texture of it can be augmented by frying it whole before slicing it to give you the chew, and the flavor can be made nice and smokey while doing so. Now I am speaking less to the ribboned style of prosciutto and more to the wedged chunks of the meat. But the delicate/toughness of the meat carries through nicely using tempeh in other foods like omelets as well. I am not sure you would get the taste from faking bacon; but the blocks of tempeh could be where it's at.

I hate to answer my own question, but I just came across this "Vegetarian Bacon" idea. Think it would work here?

http://www.flickr.com/photos/kentbrew/sets/72157624692331946/

After consultation with my mother, we agreed you could try with pine seeds.

I would suggest bell peppers. Cut them in little cubes (about the same size of pancetta pieces), then saute' them in olive oil, after a couple of minutes add some wine, salt and if you believe some spices (maybe curry). Let them cook for some 5 minutes, leave them a bit crunchy.

After that mix everything as usual :)

Absolutely dried tomatoes (South Italy ones if you can find them) - they have flavour, texture and crunchiness (after stirred in your frypan with EVO, minced garlic and chili) - you have to slightly burn them (as non-veg do with bacon) and, besides, a sauce of chopped tofu with soy milk, a dash of soy cream,  and turmeric or saffron if you prefer. Join after the sun dried tomatoes and then stir spaghetti or penne into the sauce. A good sprinkling of freshly ground pepper and ready! The appearance will be the same as the carbonara "cruel" - and also the taste strikingly similar! - Note: Cut the tomatoes into strips or cubes as they do with the bacon!

You don't really need a 1-1 substitute for bacon, but you do want something that goes well with the rich egg yolk + cheese flavor.

Zucchini has worked well for me in the past. Jamie Oliver has a Courgette carbonara recipe, just omit the meat. Make sure you brown the zucchini. (He tempers the flavor with a little cream, too.) If you can find the video, it's worth watching for his zucchini cutting technique: he quarters them lengthwise, trims off the inside corner and the cuts on the bias to yield pieces the same size and shape as penne.

Peas would also work – Mario Batali has a "Penne alla Papalina" recipe that is similar to a carbonara, but uses peas and prosciutto. (I substitute yolks for his whole eggs.)

Shiitake or another bold mushroom would likely work well, too, but I haven't tried it myself.

Also – be generous with the black pepper, and, if you use short pasta like penne, cut the veggies to the size of the pasta.

There are a wide variety of vegetarian ham and bacon substitutes. None of these are exactly the same as bacon or prosciutto, but any of them would work, though you may want to add some more oil/fat to the party, since they are generally going to be low-fat.

Here's a detailed thread on vegetarian/vegan bacon substitutes.

Bacon substitution

To summarize, you can add smokey flavor to a variety of vegetarian proteins or vegetables with a chewy or crispy texture. These include tempeh bacon, tofu bacon and coconut bacon, all of which are relatively simple to make at home. Additionally there are commercially made substitutes including Lifelife's Smart Bacon, Morningstar farms veggie bacon, Lightlife tempeh bacon strips, smoked tofu as well as various brands of Baco's or Bacon Bits. You generally add smokey flavor one of three ways - liquid smoke (easy and potent), smoked paprika (more expensive, less potent), actually smoking the ingredient in a smoker. I've also seen recipes where people smoke mushrooms and use as a smoked meat replacer.

There are also a lot of vegetarian ham substitutes including Lightlife Smart Deli, Yve's Deli Slices and meatless Canadian Bacon, and others you can find at some Asian markets.

If you want a less smokey flavor, but a similar texture you could use baked tofu (maybe flavored with miso, soy sauce and/or seaweed for umami flavor), or seitan.