OK, so people like John Gruber (Daring Fireball) are reporting that AT&T is definitely behind Apple pulling or rejecting Google Voice related apps from the iTunes App Store...

Apple Rejects Google Voice Apps From App Store

This doesn't mean you're completely locked out of using Google Voice from your iPhone though. For example, I pointed my iPhone and Windows Mobile based HTC TyTn (Windows Mobile doesn't have a Google Voice app either) at...

google.com/voice

...and saw the mobile friendly web pages you see above (iPhone on the left, Windows Mobile on the right). You can see that you can manage Google Voice voice mail from these pages. And, in fact, you can play and hear your voice mail on the iPhone. The audio file plays through Quicktime. Unfortunately, the same is not true for Windows Mobile where clicking on play results in... nothing... However, as you may note from my screenshots, you can read the voice mail text transcription. And, you can, of course, call your Google Voice number to listen to voice mail the old fashioned way.