It's becoming pretty common actually. I don't know of anyone that is doing it themselves (although I'm sure many probably do), but lots of gardeners are buying grafted plants these days. Many of the garden plant suppliers and seed companies (that also offer plants) have these in their catalogs now.

I don't remember which tomato cultivar is used mostly for this, but it's one with quite a bit of vigor and not as well liked for it's fruit. In other words, they use a cultivar known mostly for it's vigor and "toughness".

Here is a link to what appears to be a pretty good explanation of method:

Here is a link that might be useful: Washington State University Extension Fact Sheet

Looks like I should have googled to start with. Take a look at the resources on this page: WSU on Grafting Vegetables. Even detailed descriptions of rootstock cultivars is included.

The presentation linked below is full of good info.

Here is a link that might be useful: How to Graft Tomatoes and Eggplant Presentation

Perhaps you could amuse yourself by grafting red and yellow onto the same root.

Has anyone had success with grafting heirlooms onto a rootstock like that? I know you cannot plant the tomato as deep, as the grafted part needs to stay above ground. I know someone around here tried it last year and it was unsuccessful but we had a drought so who knows. I went into our local greenhouse and asked the owner what tomato they had that would be a good candidate as a rootstock for grafting and he about had a fit. He said the companies are wanting them to buy these grafted tomatoes for $5 each and that it was a scam and another way for companies to muscle in these overpriced plants and try to force the greenhouses to carry them. I would be willing to graft myself if I thought it really worked. I personally think we should be reinvigorating heirlooms with new genes to promote genetic diversity, many folks are doing this. Of course you can no longer call it a heirloom...