Google Voice for iOS has been updated to fix the crashing issue that plagued iOS 5 users on the iPhone. Users who update to version v188.8.131.521 should be able to log in without issue now. Google has also added a new feature to the app: if you've previously called a number via Google Voice, you will be able to dial it again even if you don't have a data connection. That feature works by dint of Google Voice's ability to assign an intermediary number to the actual numbers you've called, and apparently it will be stored in the app now for faster dialing.
You can get the latest version of Google Voice by heading to the iTunes link below.