Clear multiple entries form "Open With" context menu
You need to reset the Launch Services Database

Use the following command in the terminal:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user 


Wait until the terminal returns you to a prompt.  You should now be all set.

My understanding is that this will work in anything from 10.5 and up.