I asked Handspring about the power drain that springboard modules and cradles can cause in the Visor.
According to Handspring, the amount of drain that a springboard causes when it is off varies. "Well behaved" modules drain <100uA when not in use. Power management in some modules draws even less. Modules that do not use power management have drained as high as 2-3 mA when not in use. vclink will try to include battery drain information in future springboard module reviews.
Now the answer to the big question, "Is there any additional power drain when the Visor is in the cradle?" There is no power drain when the Visor is off. When the Visor is on there is power drain (up to 15 mA)because the USB chip sees the PC USB hub. Serial cradles do not have any power drain because the USB chip is off all of the time.