Also it will continue to boot into XP even after ejecting the disk until you boot into OSX via the method in the previous post and then go into System Preferences and select your Mac partition in the "Startup Disk" pane.
The option key only makes a temporary change (just for that boot cycle) so you will want to hold it down when you want to next access Bootcamp/XP.
You can also boot up in windows, select my computer click on the cd drive and then from the file menu select eject; this should eject the CD. You should then install the drivers provided by apple to get full use of your mac while using windows. In all honesty I have had XP loaded up for a while now and not once logged into it!!