I'm suprised they're just announcing it as I've been using there site for ebooks for a couple of weeks now. To answer some of the questions.
From what I've seen they sometimes list some of the formats and sometimes don't so be warned. And from what I've seen most times they have what Kobo has (I'm thinking there using Kobo's catalog).
The two formats I've seen are Adobe Digital EPUBs and also Adobe Digital PDFs. Usually they'll list the type it is but not always so be careful (from what I've seen, if they don't list the type, it's epub, but never know). So anything supporting the above you can use it on your device (or of course if you do something naughty
The really good thing I like about Borders is for most of the books you can use Border Bucks on them (unless otherwise stated) and earn Borders Bucks on them I believe (I could be wrong as I haven't bought any without using Borders Bucks so haven't earned any points towards them).
Also, be warned there software really sucks on the PC. You can't delete a book from it and the first book I got didn't load correctly and showed garbled (due to the encryption). Make sure you have Adobe Digital Editions installed before loading books into it as this is how I got them to show in the Borders app (although why use it now that you got Adobe's software? I dunno.) Mind you I haven't test PDFs so not sure, only ebooks.
And also, I haven't figured out a way to load any other types of ebooks in. Far as I know you can only load them through syncing the PC app with what you bought online. Although they do allow you to download it separately (this was how they told me to fix my issue above, to download it and somehow get it in. I didn't bother ask how to get it in as I was able to view it in Adobe Digital Editions. I couldn't figure out where they store them either.)
Hope this helps!