If anyone's still wanting Delta miles, I've had pretty good luck so far with Harris Polls. (search them on you own and it'll come up pretty easily; if I try to copy a link while I'm logged into the site, it wants to dump a referral code in the header)

Each time they send you an e-mail about new surveys, when you click on the link, it dumps you into a survey router. I've been able to click that most recent e-mail several times. Sometimes you'll get a message about no surveys matching your demographics; sometimes you can get 3-4 different surveys from that one e-mail.

Redemption downside is that they can sometimes take a little longer than '10-15 business days' after you cash out. Redemption upside is that you redeem for a Skypeso voucher through points.com rather than linking a single FF number to your Harris account, so it looks like you might be able to deposit them in different Delta accounts as needed. (They've also got US Airways dividend miles.)