You forgot that they still count data use over Microcell 3G against your data allowance even though that data is being routed over your broadband connection.
Nope. When you have a wireless connection available the iPhone firmware uses it to connect to the Internet, not the 3G.
To prove the point I just ran the Speedtest.net app on my phone at home with both the Microcell and wireless connected. I got download speeds of 10.37Mbs and uploads of .33Mbs.
Next I turned off my wireless connection and relied only on the 3G Microcell signal for data. My download speed dropped to 1.87mbs and upload dropped to .05mbs.