This is how Google Voice would work, in an ideal world: Sprint's totally integrating Google Voice with their network. What's that mean? It means you can use your existing Sprint number with Google Voice, without porting it to Google first.
Porting your number to Google is normally a mildly complicated affair, involving multiple phone numbers and other messiness, particularly when it comes to dialing out. But with Sprint, you won't have to go through that. (For instance, you won't have to dial out through an app, or get a new number for your cellphone after porting it to Google. Like I said, it's complicated.) Instead, now you can just turn your Sprint number into a super-number with Google Voice powers. And it works with basically every Sprint phone that's not the dumbest of dumbphones.
So, if you choose to, all of your voicemail will go through Google Voice's system natively (which is awesome, 'cause you can listen to it anywhere, or read transcriptions). When people call your Sprint/Google Voice number it'll ring every other phone number you'd like it to, like your home number, and you can answer from any of them. If you call or text people from Gmail, it'll show your Sprint number. Or, if you already have Google Voice, you can replace your Sprint number with your Google Voice number, netting you the same effects, but keeping the Google Voice number you'd been switching people over to.
The only slight downside of integrating Google Voice—which you can turn off anytime—is that text messages will count against you. (If you use the Google Voice app to send texts, they're sent as data, not as SMS.) But if you've got a smartphone on Sprint, you've probably got unlimited messages anyway.
All in all, this is excellent, and every carrier should be offering this same kind of integration with Google Voice, giving people more power over their number and service. But, given the way the mobile industry's going, Sprint may be the only carrier left doing adventurous stuff with their service. Hopefully not. [Google] [Full announcement]