I'm hoping that ePub becomes the new de facto-standard for ebooks, but at the time, it seems Mobipocket is. Ideally, in my opinion, you'd want a device that supports both. But, on the other hand, conversion is possible between any two given formats, even when the books are DRM-protected (Caveat: Depending on where you live, the practice of stripping DRM from a file can be illegal. Use at your own risk.).
So, to sum up, I believe you should be fine with a device that supports ePub or Mobi (that's a logical or, both sides can be true

).