Some comments.
Yes, it plays unencrypted AAC (including iTunes Plus tracks).
I beleive several people were able to use their warranty.
No, there is no rootkit in Ebook Library software. It installs a USB driver but it is necessary only for PRS-500 (I think), so you can disable/uninstall it if you want. The program itself runs completely in userspace.
Yes, you can read while charging from DC. BTW, the "most compatible" charger for Reader is the PSP one.
If you want to develop readers for custom formats, check
this.