Did you somehow delete your partition which contained Mac OS X?
When restarting, keep the alt (option) key pressed down before pressing the power button. You can also try putting a CD into the drive - holding down the option key will only display something if there are options to choose from. Putting a CD in will allow you to see if the menu comes up or not on startup (the CD acts as a 2nd bootup choice, if your mac partition doesn't exist, I'm guessing holding alt without any other option might just ignore you).
Also, try using the Bootcamp settings, which you can find in the taskbar to the bottom right of windows. Right click on the metal looking diamond shape. Then in bootcamp you can select Mac OS X as the startup disk. (From your post it seems as though you have already done this)
Finally, try inserting your Mac OS X install CD, then holding down 'C' key on startup. You can then use disk utility to make sure your Mac partition is still intact. (Google for instructions)
If the alt key thingy doesn't work when you've got a CD in the drive, then something is definitely wrong with the startup sequence. I guess you could try doing an SMC reset/PRAM reset. Again google this and it will come up with something.
Hoping for the best! I'd hate to be stuck in windows!