Given that everyone with (non-jailbroken) phones is still waiting for a Google Voice app for the iPhone, I thought I'd post how I have set up my integration even though I occasionally use the GV Mobile app. This is for people who don't want to fully switch to their Google Voice number. If that's what you're looking for, check this Lifehacker transition guide.
1) Go get Google Voice
2) Set up forwarding of incoming voicemails on your number to Google Voice