To keep the numbers, you have to force quit the field test app. Just exiting with the home button returns to the bars.
For 3.0...
Force quit an application: Press and hold the Sleep/Wake button on top of iPhone for a few seconds until a red slider appears, and then press and hold the Home button until the application quits.
For the OP: if you have the numbers/bars, just touch them until you have what you want and leave it. I think you'll have to restore to get just the bars back.
Personally, I leave the numbers up all the time because the bars are meaningless... they show good strength when the signal is weak. The numbers are more indicative of actual signal strength. They range from about -50 to -115. -60 to -80 is a good signal.