The all-things-phone-management application (which was widely speculated to have been rejected for threatening AT&T profits on calling plans) will be remade as a stylized Web site that offers everything the rejected app would have.
It is unclear if Apple would reject a repurposed Google Voice app. But earlier in the year Apple did allow Google to promote Latitude as a Web app after it was rejected from the App Store.
Considering their recent decision has managed to draw attention from a wide range of people, including some at the FCC, it probably wouldn't be the best choice to block a rethought Google Voice app.
Web apps can be bookmarked on the iPhone interface and appear like an app purchased from the App Store. You can browse available Web apps, ranging from Facebook to BPlayer (which "allows you to listen to your favorite Belgian radio stations") here.
It should be noted you can currently visit a text-heavy version of Google Voice on your iPhone by pointing Safari to google.com/voice/m.