I got this one:
App Protector Pro | Android Apps - Best Android App Reviews | Android-Apps.com
And it works perfect.
Set it up to lock any app you want and all apps that can access your SD card.
I have a widget on one of my screens for this app.
(click to turn on - instantly locks all apps you have previously selected in your settings... click to turn off - enter your password or lock pattern and its off)
Its great because you can lock the market and settings so that no one can uninstall it or disable it.. lock gallery, but still use the camera..lock messages and mail apps...
Whenever you want to hand your phone to someone, click the widget to instantly lock it down.
They can do anything on your phone, other than the apps you dont want them in. They dont belong in your gallery or astro gmail anyway.
It doesnt encrypt the sd card, but itll keep them out of it.