I use
Salling Clicker for this. It won't let you mount a bluetooth device as a storage device, but it will let you automate a number of tasks over bluetooth. For example, my system is set up to do this whenever my phone and my Mac are in range:
• start up iTunes
• set iChat to "available"
• get new mail
• check when the phone and Mac address books and calendars were last synced. If it is more than 1 week (the interval I specified), they will sync automatically.
When I make/receive a call:
• iTunes pauses
• iChat status is set to "on the phone"
When I leave my Mac:
• iTunes pauses
• iChat status is set to "Away"
Other miscellaneous tasks I perform:
• I can control iTunes, the Finder, and a number of other apps with the phone
• If I'm away from my computer but still in bluetooth range, I can activate the screen saver, lock the display, logout, or put the Mac to sleep or shut it down.
In fact, there is no need for me to have it mounted at all. I only ever need to use the bluetooth utility to download files (photos, audio) from my phone... I use scripted folders to upload files to my phone whenever it is connected, so it is all pretty much hands-off.