I am sharing an external harddisk connected to the Airport Extreme Base station. The shared storage will be used by three OSX machines as the time machine drive. So far so good.
My last-gen MBP has a 320Gb harddisk and about 265Gb of data. I am too lazy to do an initial time machine backup via a ethernet cable.
So over wifi and 10 hours later, the backup is still less than half way through.
I did not expect this scheme to work at all because my past research indicated that Apple/TimeMachine did not allow backup to network drive unless it is hosted by Time Capsule.