I was doing something like this a while ago and it roke somewhere in either Snow Leopard or Lion. I modified it and tested it both within Automator and as an application. It works but for some reason will not attach as an iCal alarm.
Kinda weird since I have atrached similar scripts to iCal recently. Perhaps you can get things going correctly.
Launch Automator and set it to create an application. Then add the following steps (actiond to the wight side of the window.
1 Find iTunes Items. Expand the box and use the drop down to set it to find Pllaylists. Add the name of the playlist to be played.
2. Start iTunes Playing
That works when run either from within Automator or as an app. The only issue is getting it to work with iCal alarms. That has been an issue from time to time with iCal. Right now on my 10.7.3 box none of the scripts are selectable as iCal alarm actions. When I get that worked out I'll post back.