There's unused code on the "Manage Your Kindle" page that (when enabled) is going to let you manage your uploaded ebooks in the exact same way as you manage any of the ebooks, periodicals, or audiobooks that you've purchased from Amazon.
And I'm not just talking about a fragment here or there; the code appears to be completely written and is waiting to be turned on. You can see a sample above.
The image above shows some of the possible attributes for a Kindle (has Wifi, Prime_ebooks, Pdocs_Archival_Support, etc). One of the attributes will track whether a given Kindle can have the personal documents backed up onto Amazon's servers.
Now, we don't know that current Kindles will get the personal document support, but it's clear that some piece of Kindle hardware will be getting that support. That's the whole point of "Pdocs_Archival_Support".
This means that there might be a Kindle that will have the equivalent of the Cloud Drive that Amazon launched a few months back. You've always been able to upload docs to the Cloud Drive but you couldn't access them from any current Kindle. That is probably going to change.
Needless to say, that is a big deal and it's arguably more important than the "prime ebook" hints. Of course, what could be even more important is that this new support might also archive your annotations as well as the ebooks. Now that would be a nice additional feature.
Everyone assumes that the Kindle Tablet will be launched on Wednesday, and it is safe to assume that the kTab will have the Cloud Drive integrated into the software. Amazon will already let you upload all sorts of files to the CD, so it would be a little silly if their own tablet couldn't read the personal Kindle ebooks that the kTab can get from the CD.
I suspect that the personal Kindle ebooks will actually be stored in your Cloud Drive and that would mean that the Kindles themselves might get better integration with the CD. I don't know for sure that this is a sign of better Cloud Drive integration, but you have to admit that it is the most likely possibility.