I'd expect some syncing to be going on when the clock/calendar rolls over to a new day, but 20 MB? No offense meant, but are you sure it's megabytes? Normally I'd suspect a 3d party app doing weird things, but it's strange that an iPhone would do the same thing.
Anyway, TrafficInfo is free (available from the Market) and should do what you want.
Edited to add: The other possibility is that the data usage is happening some other time and that's just when your provider logs it in...anyway, TrafficInfo should help.