I've had problems with the backup battery on two O2 XDA IIi PDAs.
On both occasions when removing the main battery to insert a different SIM card they've lost all RAM data.
Internal battery should provide 20 min power.
The backup facility on the ActiveSync link doesn't work properly - when I queried this with tech support they said "Oh that doesn't work very well."
O2 have just really p1$$ed me off by being obstructive when asked for an unlocking code so that I could use an Australian Vodaphone SIM for a month.
Unlocked it myself in the end.