Nethrar is a bukkit plugin to offer a Netherworld experience in SMP. Unfortunately, worldguard doesn't seem to offer an anti-ghast-destruction option like the creeper option that prevents creeper's explosions from destroying blocks (but not players).
Is there a similar possibility for ghast explosions?
I don't mind them and being (successfully) attacked is part of the game, but the destruction of my unfunished observation deck is a bit annoying...