Seems like some have a problem? -- http://support.google.com/voice/bin/answer.py?hl=en&answer=1205636
You can use Google Voice for any carrier in the US or Canada. What you need to do is give out your google voice number to everyone, and then use Google Voice to forward calls to your cell phone.
If you switch phone numbers, you just update the forwarding number in Google Voice. And since Google is in the middle of the chain, if you don't pick up a call, Google will handle the voicemail as well.
However, if you wish to only use Google Voice as your voicemail (similar to using YouMail), while keeping your current number, then you need a carrier that allows rerouting missed calls to a different number, instead of their built-in voicemail service.
Almost no prepaid services let you do this, so you couldn't use Google Voice. If you have prepaid, you must either use their horrible voicemail, or give everyone your Google Voice number and forward it to your prepaid.
From the page you link:
That would seem to indicate that prepaid phones are incompatible. I have no idea how Google would determine you have a prepaid phone though. Perhaps it's only some features that don't work, such as porting your phone number.