My roaming setup is:
1. Turn off all widgets that update themselves (OMWeather, Twitter, Facebook, RSS)
2. Set Mail for Exchange to Manual Updates
3. Turn off all IM and VoIP accounts
4. Use the 'mobile' HTML sites for GMail, Twitter, Facebook, etc.
There is no easy way to switch between my normal setup to this roaming setup and back again. So, after returning from my trip, I had to spend more time to put the desktop back to its normal state.
I recently stumbled upon 'Desktop Activity Manager' which allows you to setup different desktop profiles. This works fine for the desktop, but still requires manually setting MfE and IM/VoIP to off. I currently use 3 desktop profiles for 'normal', 'roaming' and 'car'. The car profile has only one desktop view with a few icons that are needed.
In a perfect world, while in my car, I would also like all IM to be turned off to reduce distraction, keep VoIP on so that I can receive calls, have the Phone and Maps applications started, and set the master volume to maximum. I wish I had an easy and reversible way to do this !!
Customizable profiles for the N900 would be a very useful feature for me. Not only in setting the look of the phone, but also it's functionality. Visual clues could be given to remind you what profile you are in: custom backgrounds, color schemes, or status bar icons. Switching between profiles could be accomplished via status bar icon.
No two users are alike, so any system would have to be flexible enough to allow deep customization.
Perhaps a talented Maemo developer will bring this to light someday!