Hey! I think Firstbits is a really cool idea (especially since it only relies on the block chain) and have been toying with the idea of integrating it into Instawallet. If I do, I will probably implement the algorithm myself, as to not be dependent on external services.
However, there is one thing I find a little worrying: It encourages reuse of Bitcoin addresses, since it can't work otherwise. I don't really want that catching on, especially among merchants, because I think the "proper" way of doing a sale with Bitcoin is to use a fresh address. So for that reason, I have put Firstbit support on the back burner for now. But I'll keep it in mind. Great project nevertheless!
It's not on the site yet, no. This thread is pretty much all the documentation there is. Hopefully I find some time in the next days to move it to the website. PS: I'll be looking forward to your project, I think you did a great job with Coinpal! :-)