It is interesting that you can't even buy a secure MobiPocket book when MobiPocket's server is down, even though the act of buying and the act of downloading the book are usually distinct. In contrast, Amazon still sells you a paper book if they don't have it in their warehouse, they just tell you it will take longer to arrive.
You can buy DRM-free multiformat books from fictionwise and (for science fiction) from Baen webscription
, but the majority of publishers won't release titles without DRM.
This is also another argument for buying the MS LIT version, breaking the DRM and converting it to a .mobi file. You still pay the author and the publisher (so this is ethical, even if not legal), but you are not tried down by future policies or screw-ups of a DRM factory.