Hmmmmm, if it was just your powerbook, I would have thought it might be due to something else. I have read threads about increasing the range by unplugging the battery from the tibook and pressing the plastic side against the case. This is were the antenna is and pressing it against the case makes a better contact?!?! you should use search to find more details.
But since you got the problem with both books at the same time, I think you could really blame it on the update, eventhough everything is the same here after the update. For ibook and tibook...