Why haven't I seen any ghasts in the nether?
Get out in the open
Start digging tunnels and look for a BIG AREA (the ones that involve lots of lava).
A Ghast can spawn in a small area but the chance is very small compared to a big area...
If you generated the world before the 1.2 patch, there is a chance of them spawning above bedrock, assuming you have been there.