About 225 747 freighters operate around the world, and those numbers (as a market) took nearly 30 years to develop.
Currently (I just checked) there are 17 -400ERs on order, 6 -400ERs for Quantas, and 11 -400ERFs for various customers. Looks like the real interest is in the freighter model rather than the passenger model, but given the current post 9-11 environment, that's understandable.
(edited for a typo)