I have actually had the EXACT same problem and was shocked that nobody else was talking about it. Hopefully iTunes 7.5 will fix this.
One thing is for certain, I was expecting some software fixes by now. The iMac fix apparently only applies to the new iMacs. But I am having a few strange problems such as the drag and drop issues in iTunes that are plaguing the otherwise great Leopard.
I got the same problems here... even if I try to drop songs into a folder on the desktop. I installed all available updates, but the problem is still present. The strange thing about that is that the drag and drop works sometimes but only when dragging one song.
My iTunes library is stored on a network volume (AFP)... is this the problem / bug?