Something is off somewhere as it was working with Growl 1.3 for a couple of days without issue... but yesterday it stopped on my MacBook Air.
I can no longer select "Growl" for "visual Notifications" in the "Notifications" menu in Skype's Preferences. It's greyed-out.
Exactly the same problem for me: installed Growl 1.3 and it worked fine but after the first reboot of my laptop it stopped working. Skype is still listed as a registered app in Growl but it is greyed out in Skype settings so can't be selected.
Mac OS/X Lion version 10.7.1