Here's the steps I took to change MicroB's user agent to the latest iPhone:

Download "Hide User Agent" from Extras-Devel (Usual warning applies!)

Launch the app from the settings menu and click "User Agent." Use the following setting:

Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_1_3 like Mac OS X; fr-fr) AppleWebKit/528.18 (KHTML, like Gecko) Version/4.0 Mobile/7E18 Safari/528.16

When you want to switch back to MicroB, just click on "remove" within the "Hide User Agent" app.

I tried this to use the web-based Google Voice app for the iPhone. It sort of worked- but the formatting was all messed up-- although I could dial out and my google voice # showed up in their caller id. Not pretty, though.