If you're like most heavy Google Voice users, you keep your GV voicemail and standard carrier voicemail separate. This is great for managing different numbers, but it's a bit of a pain to wrangle all on the same phone. Android and Google Voice get a little closer today, as GV's voicemail service is available from the native Android phone/dialer app. The update treats voicemails like calls, so you'll see them in the same place that you usually see Google Voice incoming and outgoing calls in the dialer. Download the updated Google Voice app to apply the changes.
Unfortunately, this feature is limited to Android 4.0 at the moment. Google may back-port the voicemail integration into an earlier version, but that seems unlikely - if I had to guess, I'd say that this is something Google's been planning for a while and had to code for specifically in Ice Cream Sandwich. On top of that, early reports indicate that phones without a mostly stock Android system can't use the feature; skins like Samsung's TouchWiz and HTC's Sense UI usually completely replace the dialer and contact system. HTC Vivid users with their brand new Android 4.0 update are reporting that the feature doesn't work for them. Another new feature that's exclusive to ICS (but probably works for all Android 4.0 builds) is access to Google Voice folders from the app itself.
The addition is a handy one for the admittedly small intersection of Google Voice users and stock Ice Cream Sandwich (or a suitably close ROM) users. While this isn't the VOIP addition to Google Voice that many have been hoping for, it's one more way that your Google Voice number and identity can easily move between Android devices. There's just no way to match this kind of integration on another platform - something that Google's surely proud of.