According to an employee in the Google Support Forums, Google has identified a possible software issue that may be causing the 3G connectivity issues on the Google Nexus One. They go on to state that a software patch is being tested and the initial results a positive. Expect an Over The Air update to hit your devices in the next couple of weeks.
Obviously this will not help the problem if your in a poor 3G reception area. You can also change your phone to be always in 3G mode as that has been stated to make a difference to the disconnect issues some people have experienced.
You can also try the solution posted by jkOnTheRun and see if that helps you out.
