I don't know if it only happens to me but I can only find pigman. Could it be a bug or certain seeds don't spawn ghasts?
I've tried looking for them around the nether and have not found any.
FYI - My server.properties option for monster spawn is set to true. No peaceful mode is off and it's a SMP map.
How could that happen?
SCREENSHOT AS REQUESTED
So as you can see only pigman in sight.