I have the same problem.
It seems to be a system problem instead of a trackpad problem. I keep having problems with dropping in general.
I work with a Wacom Bamboo, but the problem has been occuring with the trackpad, mouse, wacom tablet and remote desktop software such as Logmein and Mobile Mouse.
The problem appears only some times, but when it does it happens with every single drop.
Also it happens to me with files that I drag from finder, text that I drag from any application and files that I drag from google chrome (files that I downloaded) to finder windows.
I have not experienced any such thing in Snow Leopard and experienced it numerous times in Lion, so I assume this is a Lion problem.