I was able to register for the Sony Rewards site late Friday evening (so about 60 hours after I was approved for the Sony Card). I might have screwed up my chances of automatically getting the $50 part of the $150 though.
When I first logged in I had 50 rewards points. Instead of just adding a PRS-505 to my cart and checking out, I accidentally added it to my wish-list. The first time you add something to your wish-list you get an extra 50 rewards points. So now I had 100 points, which made my order $298.99, which was under the "$299 or over" first purchased required to get the $100 part of the $150 (and presumably the following $200 for using the Sony Card for 6 months). So I called Sony Rewards customer service and the helpful CSR showed me how to tie up 50 of my points in some "spin the wheel for a prize drawing" feature on the site and bring my purchase price back up above $299.
However, the CSR was concerned that I wasn't being charged $2 for shipping and handling. I pointed out that the site says S&H was included, but he said the $2 S&H was supposed to be automatically added for the PRS-505 deals to insure the price would be over $299 when you ordered. He made some notes in my account explaining that I was making this purchase for the $150 credit deal in case I need to call back again later.
So now I'm waiting for my reader (should ship tomorrow) and hoping I don't have to call back in 12 weeks to ask about my $50 credit. Very excited about the reader though

(and still wondering if I should have sprung for a PRS-700

).