The Google Voice app for iPhone was updated today with support for the iPad and iPod touch. According to the official Google Voice blog, the update will let your iPod touch or iPad send and receive messages and enjoy other features of the service, except making calls.
However, you can use your iPad or iPod touch running Google Voice to initiate a call on your iPhone with something called Click2Call. The way it works is simple. Just select the number you'd like to call, and then tap the phone you'd like to use. That phone will ring, and when you pick up, Google Voice will complete the call. I don't know why you wouldn't just use the iPhone in the first place, but there it is.
There are some other cool features as well. For instance, enabling push notifications disables text forwarding, so you don't receive duplicate messages. Plus, a new "Do Not Disturb" setting sends callers straight to voicemail. Finally, there's a new Contacts button in the dialer tab for easy access, and texting is quicker with the removal of the OK button.
It sounds like a nice update and you can grab it here.