App to allow me to set which contacts dial out with Google Voice by default? I'd like to be able to set, at the contact level, which phone number is used to dial out.  For example, I'd like to be able to set it so calls to my boss use Google Voice by default where as calls to my family use my cell number.  Any suggestions?
I still haven't tried Voice Choice, but I just downloaded this free app: Voice Plus

This looks perfect.  From the description:

Allows you to set rules for which number dialed will use your Google Voice number. Specify rules based on area code or by contacts (using contact groups). No more annoyance of having to press another button when dialing to choose where to call from!