I think a lot of people, including me would love this. It may become a feature of Google Voice down the road - Google has all the parts, they just need to be linked up.
Google Voice in combination with the Android app "Locale" gets you really close too. A number of Locale users have put this on the Locale wish list, but so far nothing.
The developer of Google Voice alternative 'GV', was working on a plugin to make this happen with GV and Locale. Unfortunately GV recently stopped development.
In my case, T-Mobile coverage at my home is terrible and unusable. But, I didn't want to wait for Verizon or AT&T, and wanted an Android phone so bad that I went with T-Mobile anyway. Google Voice helps with this issue a lot, but I would like to have my Google Voice stop forwarding to my land line, and forward to my Android automatically when I leave the house and coverage is better.
Oh well, I'm sure this will happen soon...
-Slosuenos