Possibly stupid question: Can a Bluetooth-enabled cellphone itself be used as a Bluetooth headset for Skype on a Mac?

I tried, but couldn't see my Motorola W385 as a headset in:
Skype > Preferences > Audio > the Audio output, Audio input and Ringing popup menu lists.

No, that would not work, as Bluetooth is not a peer-to-peer protocol. In Bluetooth, everything is either a Bluetooth host or a Bluetooth device. As both the cellphone and the computer are Bluetooth hosts, they need to communicate to a Bluetooth device, and cannot talk to each other.

"Competing" technologies, like Zigbee, create a peer to peer network, where different nodes on the network can use each others' resources. If Bluetooth had used a topology like that, then the answer probably would have been 'yes'.

Trevor