We had an ice cream cart come to our wedding, and it sounds like it was quite common for our vendor (the famous Jeni's). Check with local ice cream shops and see if they have some sort of set up to take to fairs and etc. Our ice cream came to approximately the price of a wedding cake for the same amount of people.

bride and groom ice cream scooping!
http://www.flickr.com/photos/stanzi/6555466283/in/set-72157628488998129

I love this idea! Depending on your venue and/or the size of the wedding, a sundae bar could be fun, with your ice cream flavor and complimentary toppings. If its laid out well and perhaps has someone helping serve, it could be very classy. I think all those flavors together would be wonderful. A quick note on your custom flavor, if you're unsure of any of your guests' food allergies, it might be a good idea to provide an alternative to your custom flavor for people who may have chocolate or peanut allergies.

Fancy flavors - raspberry jalapeno or raspberry cabernet. You could also do a peanut butter & jelly flavor or something else fun. Ditto peachy44 on the peanut allergies - make sure you have something peanut free!

ice cream cake! not sure where you are but bi-rite creamery in San Francisco makes a really good one.

I'm with kestrel on the raspberry cabernet, perhaps with a dark chocolate swirl or chip. I'd keep the peanut butter separate for allergy concerns and consider pairing it with some pretzel chunks.

Really how to serve it depends on the rest of the meal, the venue, and the number of guests. For a seated dinner with a large group, i think individual ice cream bombes would be great and very elegant.

If you drip some chocolate or berry ice cream on to your dress, is it going to ruin your day? Get a flavor that won't ruin the dress. Vanilla lavender would be summery and non-staining.

We're having gelato for our rooftop August wedding. Cakes and cake-cutting fees are SO expensive, and we figured everyone'd want a cool treat for dessert. However, depending on where you're having your reception, some reception locations only allow food from specific contracted vendors. If you're having in your backyard or somewhere like that, it probably won't be an issue.

I'd go with peanut butter and chocolate, chocolate and raspberry, and plain raspberry sorbet to give everyone options (peanut-free, chocolate free, and vegan are all covered by those three flavors). Dress it up a bit by serving it in fancy bowls or by sprinkling it with some pretty toppings: fresh raspberries for the chocolate-raspberry and the sorbet, hot fudge or tiny chocolate & peanut butter chips for the peanut butter-chocolate, and maybe homemade whipped cream on the side?

Sparkly raspberries (rolled in sparkling sugar then frozen) or sugared violets to top it would be pretty
http://www.spicesherpa.com/2010/05/06/sugared-flowers/

Or make it gourmet by topping it with cacao nibs mixed with chocolate curls

Not sure how easy it would be to find a vendor who could do this, but gelato flowers would be fun, memorable and beautiful. I'm most familiar with Amorino, I believe their only US location is in the US though.

we also wanted to serve a cool treat for our wedding coming up this August. we'll be serving gelato popsicles! (easier than scooping each item individually)
http://bargelato.com/

What about something different like a praline semi freddo - you could get the praline made from peanuts... goes really nicely with Chivas Regal Icecream as well..