I was really lucky in that I got a Gizmo5 account shortly before they were acquired by google. They created an Android app called Guava that did just that (limited to 3 minutes at a time unless you pay). It comes in handy when I'm traveling internationally as I can make free 3 minute phone calls to the US.
Since Google owns Gizmo5 now and I'm assuming that this new browser calling feature is a result of the acquisition I wouldn't be surprised if an Android app is going to be out soon.
IIRC, Guava was actually based on Sipdroid, which is under active development. If you pair your Gizmo5 number with your Google Voice account, and sign into your Gizmo5 account with Sipdroid, you can easily make free Google Voice calls on your device. It's just a little roundabout. For outgoing calls, I think you have to initiate the via Google Voice's web interface, then answer the connecting call with Sipdroid. Works like a charm.
I'm hoping so. There are many times when I have sucky T-Mobile coverage but have access to wifi (like when I'm in the back corner of our building :-O) where this would be perfect.
I hope so for several reasons. One being that I don't normally have gmail open (I prefer thunderbird/IMAP + an XMPP app for IM), but my Nexus One is always on.
