...and I don't time-machine my OS. Is there any other way to get screen sharing back?

Which update for which OS? Leopard, or Snow Leopard?

Either way all you need do is open up Sys Prefs and go into the "sharing" menu and select anything you want to share.

If "Screen Sharing" is already checked then you've got another problem but the functionality works perfectly from 10.5.0 all the way to 10.6.2.

Thanks Robert. I found a different way to open screen sharing and it's working beautifully. Thanks again!