I've been wondering this a bit too.
I was recommended this site as a bit of a guide for readinesshttp://www.pantley.com/elizabeth/books/007...isbn=0071476903
DD is a little younger but has started telling me when she has done a wee in her nappy (or perhaps just as she is doing the wee) but I haven't tried to put her on a potty yet. She hates doing a poo and cries every time so if I was a bit more dedicated, I could probably put her on the toilet for that every time. Sometimes when I've done that though, she hasn't ended up going and waits til later to poo, but mostly she has ended up going in the toilet.
At the moment, I am thinking I will start with some books, setting up a potty (as well as the little seat on the toilet) and seeing if she seems interested, but I want her to score a few more c's on that survey first. I figure if she is showing signs of readiness, then it's worth a shot. If it doesn't work, well, we try it again later and if it does, think of all the nappies that I didn't have to change.