Not sure here, but I guess that the initial power surge of turning your Z on as well as the default ac adapter setting of full backlight drops the battery below full causing the charge light to come on. Depending on your ac supply, especially if you are only drawing 100V in US as apposed to 110V in Japan, the ac adapter may not be giving enough juice to push the battery over the 100% mark.
I\'m sure that, like lardman, my charge light eventually went out but it was a long time after putting the Z on ac adapter - I was also copying al ot of files to/from CF and SD so that is probably using energy too.
Stu