A solution is here:
http://forums.obdev.at/viewtopic.php?f=4&t=3305&p=11229&hilit=screen+sharing#p11229

I.e., if you use LaunchBar (or a similar utility), invoke it with a key stroke (I use control-space). This effectively switches you out of Screen Sharing and into Launch Bar. Then you can hit command-tab and it will operate locally; Screen Sharing will obviously not be able to capture the key stroke because Screen Sharing will be running in the background (and is not designed to capture keystrokes then) so the window switching utility will be invoked. All this without using a mouse, which is the whole point.

Anyone concerned about the issue in this post should be using LaunchBar or something similar. It's an indispensable OS X productivity tool for people who prefer to get things done fast via the keyboard rather than slowly RSI themselves with a mouse.
see:
http://www.obdev.at/products/launchbar/index.html