You may PUG at any time:
10-man
Naxxramas
Obsidian Sanctum
Eye of Eternity
Vault of Archevon
25-man
Naxxramas
Obsidian Sanctum
Eye of Eternity
You may PUG AFTER SUNDAY'S RAID HAS STARTED
10-man
Ulduar
Trial of the Crusader
Trial of the Grand Crusader
Onyxia's Lair
Icecrown Citadel
25-man
Vault of Archevon
You may NOT PUG at ANY TIME:
25-man
Trial of the Crusader
Trial of the Grand Crusader
Vault of Archevon
Onyxia's Lair
Icecrown Citadel
Remember, if in any doubt at all, ask an officer before pugging any raid.