I have used my 2009 MBP with two external DVI screens, one connected via the Apple dongle connecting to the DisplayPort connector and another via a USB to HDMI adaptor. The MBP remains closed up and have Apple Bluetooth keyboard and Magic Touch trackpad.
The USB connected screen is noticeably slower to update, but that isn't a problem if you use the DisplayPort connected screen as the "main" screen.
The battery might be affected in the long-term because you need to have power connected in order to use the MBP in closed up position. Sometimes it doesn't start up correctly for me, so I have to open the MBP up to start and then close up to go back to work. My desk is rather small and leaving the MBP open is practical in my case.
Overall this arrangement worked well for a couple of years. I have since bought a mini-tower and direct connect both screens. Of course, I now also use Linux, but that's another story about leaving OS X for Linux.