Same problem here and as a previous poster mentioned, I too have seen a pattern to this. Latest version of iTunes (10.x), two iPhones (one iOS 4.x, one iOS 3.x), two computers (laptop & desktop), ONE iTunes account. Have never had this issue. Almost everytime I make a purchase whether free or otherwise it requires me to verify the CC on file (I have to enter the security code each tme) due to "a new device has made a purchase". What's new? All 4 have been on the same ID for months.
Changing cards ... no go. Changing password ... no go. Deauthorizing all and reauthorizing ... no go. Called my CC company (both) and neither said there was a security hold, etc and that each authorization went through just fine. They researched all Apple transactions and none have ever been blocked. Credit Card Security ... not the problem.
Here's what triggers it for me ... I download a free or purchased item from iTunes on one device. I am prompted to verify. I comply and enter my CC security code again. From that point on any purchases (free or otherwise) from that particular device are honored without exception.
I attempt a purchase from another device on the same account and it requests verification. I comply and thereafter all purchases are honored without exception. I purchase from another device and BAM it happens again so it appears as if Apple is only storing purchase validity for ONE device only even if there are multiple on the same account. I never had this issue until I updated the iTunes software about two weeks ago - I was happily downloading free and $$ items (apps, music, videos, etc) from all devices without having to verify each time I switched to a new device.
All persons I know with multiple devices connected to their iTunes account are experiencing the same behavior. The workaround ... only use one device. Yeah. Not very realistic in this day and age especially when everyone is touting the "cloud". Seems to be a bit of stormy weather for Apple with this particular item. I'm sure they'll eventually correct it but for now it's simply a pain in the tucas but not an issue that I would cancel over. Hey Apple: perhaps a few free downloads for this inconvenience are in order??