Encrypted files formats (e.g. files with DRM) are not transferable, and most new books are only availabe with DRM. Fictionwise supports several formats and has a good
reading software and devices FAQ. There are some formats not coverred by Fictionwise. EbookMall has a similar
Device Comparison with a few more formats. The Sony Reader only reads DRM e-books from the Sony store.
If you don't have any e-books yet, the nearest thing to a "future proof" e-book with DRM is probably MobiPocket. This is already supported on many devices, and MobiPocket is now part of Amazon. The iLiad is expensive, but it includes MobiPocket Reader and also FBReader (which reads many kinds of e-books without DRM). Otherwise your MobiPocket choices will come with a LCD (rather then E-ink) screen. However, the Amazon Kindle (if it is ever made commercially available) will be a less costly MobiPocket compatible E-Ink device.