I've found that going into the Control Panel -> Display applet and modifying the dim interval (ie. change your current "Dim" interval to something else and then change it back to your preferred interval, then click OK) will get the display dimming again.

Not ideal, but better than a reboot.

I'm not holding my breath for an early fix to this problem either.

Vote it up in Bugzilla if you have this problem (that means all of you using Skype! )