The default setting in the GIMP has a mouse pointer highlighter active. It makes working in GIMP difficult- to the point that it's been mistaken for a bug. I came across this issue before- and disabled the highlighter immediately. Using another computer today, I came across the same issue, but I'd forgotten how I'd fixed it- and it took me a while to find the solution. Eventually I found it on Pimp my GIMP.
Go to File>Preferences>Image Windows and untick Show brush outline.
Update: this looks like a Debian Lenny issue: my Debian Squeeze machine doesn't have this problem. Time to update the old laptop I was using when I wrote this post, probably.