Well... Morpheus is here for the Mac... sorta...
As previously reported - Morpheus was considering making a switch to the Gnutella network - and today released a Preview Edition (which has 53 million downloads at the time of this post)
What this effectively means is that many Morpheus users will be switching to the Gnutella network - providing many more users/files... so if we can't bring the Mac to the Network, this basically brings the network to within the Mac's reach.
While there is no official Mac client... any of the current Gnutella clients will be able to access the same files. Many people like Limewire, however an OS X native client, Aquisition is an up and coming client with a lot of promise.