You can try this on your N800 yourself. This is how I did it (I didn't touch the opera config file, but that could work as well, but presumably for all sites instead of specific ones):
- Install privoxy
- Set up a network config version that uses proxy (127.0.0.1 port 8118)
- Append the following three lines to /etc/privoxy/user.action:

{+hide-user-agent{Mozilla/5.0 (X11; U; Linux i686; rv:1.8.1.1) Gecko/20061205}}
.google.com
www.showmyip.com

(it doesn't have to be _exactly_ that user-agent, I just used what www.showmyip.com showed me when I pointed my desktop Firefox to it)

- Do /etc/init.d/privoxy stop
- Do /etc/init.d/privoxy start

- Test your setup by pointing the N800 to www.showmyip.com, the user-agent is down there somewhere.
- If it looks ok, start testing google apps.

(Obviously some of the steps above have to be done as root)