Well, there's Ubuntu, I suppose.
There's supposed to be a crop of mobiles coming out with Ubuntu's smartphone variant in the next few months or so. However, I doubt that you'll get a completely Open Source OS on a smartphone any time soon, whether pre-brewed (like Ubuntu) or home-brewed. The reason is that most (perhaps all) of them contain processors or chipsets whose APIs aren't available without a Non Disclosure Agreement. You either have to put up with object-code-only drivers, or you don't get to use the neat stuff - or perhaps not use the kit at all. After all, if you don't have access to the video processor APIs, you're not going to be using the screen, and that's not exactly compatible with a smartphone OS!
That's not to say it's not possible. I'm hearing rumours of fully open handset hardware being created in India, for instance, and that would be ideal for a bare-metal Open Source OS implementation. Could be an interesting crowdsource project!