From some point, after years of using gimp, I found out that it has a strange bug when trying to drag a new guide from the ruler or an existing guide from a previously saved file.
At this very point, the mouse started to lose responsiveness as if it was being blocked by some hidden layer. Also, I noticed that none of the hotkeys were also working, so everything looked like all hanged.
After some googling, I found out that killing xscreensaver would fix the problem.
killall -9 xscreensaver
Reload gimp, and dragging the guides will work as before.
Works for me under Fedora 13 and 14.