I first found this problem when using ubuntu dapper & emacs 23.0.0.1.

Sometimes when speedbar is started in emacs, every key press in emacs does not effects unless next time I switched to emacs.

At first, I thought it was a little bug with emacs 23, so I did not take it serious.

But after upgrading to feisty & emacs 23.0.0.9, this happens more often. When created a new frame, either speedbar or C+X 5 2, key press frozen. And everything returns all right when all other frames are closed. And evem more, key press lags when switching windows! So I thought it maybe bug with xorg or sawfish!
So I asked in emacs@newsmth, and someone said it maybe problem between xim & emacs in reply. Then I disabled scim and now seems everything ok.