Proficient coder and mapper metlslime has released a new tool to use skip on maps. It's supposed to support more maps than Tyrann's skippy and also supports skip on liquid brushes. Readme - Download - Source and more infos: Func_Msgboard
I guess you could create 'glass' with this. it is better than clip because weapons are still blocked, and better than the trigger entity glass hack because it wont bleed.
For example you can use a skip brush for walls like this _____/^^^^^^\___ when you do not want to have the player get into the empty room (can be stupid in deathmatch) like this _____/^^x^^^\___. With skip you can make the space where the player can walk into like this ________________. No crampyness, but smooth soft dm goodness.
Basically just an invisible brush that blocks the player.