Cool, thanks for the advice, everybody. I've decided to go ahead with it, mostly for the reasons that Mara, Cameron and myself mentioned.
There's another reason that I failed to mention -- the lead role (the only one I'm really concerned about casting now) is for that of a college student, and there's a good chance that I might be casting an actual college student. In a month from now, many college students will be going home for the Summer, and I don't want to wait until they return, in the Fall, to start casting this role.
Kholi, you're correct to point out the fact that I don't need to book, and maybe just start feeling people out who are interested.
I think what I'll do is to address an issue that Rik pointed out -- when casting, I'm going to be straight-forward with them and let them know that if some big break or really huge opportunity arises, between now and then, that I won't hold them to their commitment. Truth be told, though, I won't be casting any "names", and for most of the people I'll be looking at, this movie is likely the closest thing they've got to a "big break", or whatever.
Thanks again!