I am having the same issue. For me, 10.6.6 has been rather sluggish in general.
Spent a few minutes raging over the fact angry birds didn't download for me. read this thread and: i tried the itunes single of the week thing, accepting the new permissions and now it works fine!
So i assume apple just figures everyone on the app store also buys itunes things a lot
I had the same issue. I opened iTunes & accepted the new EULA. Rebooted. Then everything worked great. Hope this helps anyone.