I have Google Voice set up (kind of) so that I can use Handcent with my Google Voice number. This set up works if you have the patience. But its worth it if you have a text plan that you'd be paying extra for.
1. Text a contact from Google Voice; you can use either the phone app or the computer app.
2. When they reply and you get the message through your non-GV messaging client, they will have a ten digit number assigned to them that is not their phone number.
3. Save this separate number as OTHER in that contact, or whatever is most convenient.
4. When you text that number from Handcent, Chomp, or your default messaging client, it sends them texts from your Google Voice number.
This method is the only method that works, and it does work, if you don't want to use the GV app and you text alot. Two things to keep in mind:
1. The separate number that you save for them is not in anyway their phone number, it seems like a routing number, but it works every time.
2. It takes a little patience, but worth it to be able to use Handcent + Google Voice for free texts.