The nice thing about MobiPocket is that they have a version of the reader for practically everything with a microprocessor in it. If you have any type of PDA, there's a probably a MobiPocket Reader for it, so you don't have to wait until you get an iLiad (excellent though it is!).
A site I'd thoroughly recommend, in addition to MobiPocket themselves, is
FictionWise. They offer a somewhat different selection of books to MobiPocket and have lots of "special offers". They also, as I mentioned earlier, have a large selection of books which don't have DRM.