I was just having this problem, I couldn't drag mp3s or playlists into itunes at all (I would just get the black crossed-out circle) even though I did it yesterday. I found that, for me, it was related to whether itunes had been manually opened or automatically popped up.
When I plug my ipod into my computer, and itunes automatically comes up, I can not drag anything into itunes. If I manually open itunes from the start menu, I can drag songs and playlists into itunes.
I have my ipod set to manually manage music. I can open itunes first, then plug in the ipod, and be fine. I can plug in the ipod first, close the itunes that automatically popped up, reopen it from the start menu, and be fine. I'm on Windows 7 64-bit, iTunes 10.6.1.7