The saga continues.
As someone who purchased RSP just days before the blankety-blank Pixmantec sell-out, I too got an e-mail, clicked on the supplied link (using Firefox), got to a page that said your serial number is [blank] and your platform is Windows. Please record your serial number; you will need it later.
Strangely, [blank] does not prove to be a valid Lightroom serial number.
After 10 minutes navigating Adobe's telephone support - Kenny G over a monoral land line is apparently thought by some demented soul to in some inscruable way lessen the pain - I learned that this is a known problem and that this known problem will be addressed by some unknown form of communication, possibly a second e-mail.
If you encounter the same problem don't bother clicking on the refresh button or trying a different browser. Perhaps the ONE thing Adobe is competent at is protecting its own interests. The moral of the story is that if you are an RSP owner expecting the free upgrade and you get the tantalizing email, better yet -
WAIT UNTIL TOMORROW.