Not really, no.
For one thing, one of the leading publishers of SF and Fantasy, Baen, produce books in open formats which can be read on any device.
For DRM-protected eBooks you have two main choices if you want a wide selection of books: MobiPocket and Microsoft Reader (LIT). MobiPocket books can be read on a wide range of different devices, from "smartphones" to PDAs to PCs to the iRex iLiad to the forthcoming Bookeen CyBook. LIT books can only be read on Microsoft Reader, but depending on where you live it may be legal for you to "crack" the DRM with a free program that you can download, and hence convert the file to any other format that you wish (eg for the Sony Reader).
So basically my advice would be to choose the device first and foremost. You'll always been able to get content for it.