I am having exactly the same problems on all 3 of my devices. Just got off the phone to Apple, spent an hour resetting my router, iPad 2 and iPhone 4S and going through several iMac (Which is Ethernet Connected) troubleshoots and still got nowhere. iTunes can have 20 downloads queueing and either download none or download 1,2 maybe 3 then dump the rest with error -3150. Apple Care conceeded it could be a server or account issue and advised me to contact iTunes support via apple.com.
I read an article online today about a guy who was having "cannot connect to iTunes store" issues on iPad and after a long drawn out process, was contacted by iTunes support who said he had several "downloads pending" on his account at their end and they cleared these and this solved the problem. The connection problem is isolated to iTunes on all 3 devices so i'm convinced it's a server or account issue. Will post again when I get a response from iTunes support.