We have our set up configured as azrobert describes.
Google voice mail will always intercept the call. While callcentric's vm might pick up, all you'll end up with is silence in the recorded message.
There really is no perfect solution to this unless you're running a pbx which is answering all inbound calls (and routing appropriately) so that google voice mail can't intercept the call. This isn't a bad option but adds another layer of complexity and point of failure. In addition, the problem with this is you lose gv's transcription ability and syncing the voice message with android's google voice app.
I decided the best set up here was to have google voice mail email the voice message along with attachment to a pc that's almost always on during the day. In addition, the google account used for this line is set up as an additional account on an android cell phone.
On occasional there is some voice mail confusion between the answering machine and google voice, but it's minimum and something we can live with. Personally I'd like to do away with the answering machine altogether, but other household members don't see it that way