due to the portability of my X21, i hardly use my T42 often.

is there any harm in leaving it plugged in if the machine is to only be used "once in a while" --what sort of battery drain can i expect if i leave it unplugged (i have a 6cell and the ultrabay slim)

thanks.

How often do you use it? Once a week, once a month....other frequencies? I've read before that if a Lion battery is to be maintained without being used for awhile, it likes to be at about an 80% charge. I would drain it to 80% and take it out of the computer all together if you don't use it but once or twice a month...Re-insert it when you use your Thinkpad...or if you only use with AC power, then don't insert it. Just check your battery and charge it to 100% when you do, about once a month and let it drain back to 80% again before re-storing it....Also, occassionally these batteries like to be drained down to the point of the computer giving the signal that they will be turning off. Maybe every other month you should do this...but re-store at 80%...just my opinion.

its being used about once a week at the moment, its been plugged in all the time.

would it be better for me to leave it unplugged and just use it when i need to, after which i'll charge it and unplug it again?

Once a week....then I would leave battery in and just unplug it when not being used.