Sony just announced a definite date for the Epub switchover. The announcement also mentioned that the Sony Ebook Store will be renamed to the Reader Store, with a new site address:
http://readerstore.sony.com.
Next week will also bring an update to the Sony Library software. v3.1 will add support for Windows 7 and Mac OSX 10.6.
As of 11 December, LRX files will no longer be offered for download. The only files available will be DRMed Epub. If you're one of the PRS-500 who hasn't upgraded to the Epub firmware,
back up your ebooks before it's too late.
Thanks to Anemic Oak for also reporting this!