I've used Aldiko and have actually been very pleased with it on my phone. The EPUB format seems to be somewhat superior as I've now started to delve into and learn more about e-books.
For whatever reason, I figured once I got the tablet I should go with something more "legit" that I can use to check books out from my local library, etc.
Since this has become such an issue, though, I may indeed decide to stick with Aldiko. I don't have that much investment into either borrowed or purchased books yet with the Kindle app.
From some other posts on this forum it sounds like the solution might be to convert from MOBI to MOBI and that somehow this will add the thumbnail image that seems to be missing.
Either way, it doesn't appear that Calibre is going to be able to manage my library on the tablet itself so I'll still be stuck with a multi-step manual file transfer process. But it still seems to be a good tool for managing my library in a central location and its conversion utilities.
The bulk convert utility (EPUB to MOBI) seems to fairly consistently crash my Windows 7 laptop, however...