VALVe wanted beta testers to only play on de_dust for the beta, user made maps will not report back propper gameplay statistics to VALVe. It seems there must be some sort of in game development mode in HL2 to make the maps. Also Source comes out this week, before HL2 so I gather there shouldn't be any custom maps until HL2 is officially released.
http://www.steampowered.com/forums/s...hreadid=127532