I've read somewhere that, by default, it's not possible to share an airport internet connection to a bluetooth device, while it's possible to share an ethernet or adsl port.
Somebody suggested buying a firewall/gateway software but indeed it's simpler than that.
The following example has been tested with a treo 650 and OS 10.3.8, a sitecom Bluetooth and Airport extreme; it should work on other versions too:
1) Assign a local IP to your bluetooth device (in this example: 10.0.1.105)
2) Pair your pda via bluetooth to your imac; after that, the bluetooth discovery option can be set to "off"
3) Open a terminal window
4) Run a ppp daemon without authentication on the pda bluetooth port (the following command all on one line):
sudo /usr/sbin/pppd /dev/tty.Bluetooth-PDA-Sync 115200 noauth local passive proxyarp ktune asyncmap 0 persist :10.0.1.105
n.b. the "persist" flag will keep the daemon running even after a connection is terminated, waiting for new connections
4) Enable ip forwarding:
sudo sysctl -w net.inet.ip.forwarding=1
5) Setup your PDA for a new bluetooth connection; the device should be setup as "pc connection", 115200 spped, and a fixed IP (in our case 10.0.1.105) and DNS same as the one on your Mac