How do I pair my iPhone with my iMac with Bluetooth?

Without knowing what iMac you have I can’t say if it doable. My iMac doesn’t support Bluetooth. Yours might not too.

Go into Applications/Utilities/ and fire up Bluetooth File Exchange and see if it give you a error.

I connect a Nokia phone, a bluetooth headset and a wireless Mighty Mouse to my MacBook Pro.
If you have built in bluetooth the procedure should be simple as can be,

First – make sure the bluetooth icon is present in your menu bar: go to System Prefs. > Bluetooth > and click “Show Bluetooth status in the menu bar”
Second – make sure bluetooth is switched “ON” on both the computer and phone, and that they are discoverable.

Then you can go ahead and set it up: go to the bluetooth icon in your menu bar and select “setup bluetooth device” from the dropdown menu. The wizar wild guide you through some steps and you’re done.

You can’t do much.
iPhone doesn’t support the file transfer that @Inofaith is talking about.

I’m pretty sure you can’t use it like a microphone.
You can’t sync music/contacts/apps/anything with it (no iTunes, syncing is wire only)

@johnpowell Most, if not all, post 2006ish Macs have bluetooth

@johnpowell
Try adding your iphone to iSync

its supposed to update contacts and calenders

Bluetooth on the iPhone is right now only good for connecting to a headset. All syncing is done either through iTunes with a USB connection or via WiFi through MobileMe (awesome) or a Microsoft Exchange server.