This is a topic that I've received a number of questions about how to do, and it's easier than you might think. The hard part is figuring out your network providers service plans and settings, which I hope layout step-by-step here...

What I'm going to leave for another time is price breakdowns of data plans for the major network providers (here in the US).  It's complicated and truthfully, it changes based upon the network rep you get on any given day. Just keep in mind that if you're not signed up with a data plan, or even just a regular "WAP" plan (the service that you use to surf the net on your phone) with your service provider, using your cell phone as a wireless modem could be very costly.

FYI: the phone I'm using for this howto is the Nokia 6620, and I'm using Cingular as my provider, so the settings that may be depicted in the images may not work for your phone or provider, but you should see settings for your phone and provider below.  So, let's start off with the setup, shall we?  

Grab the Modem Scripts:

Mac OS X comes with a lot of stuff built in.  Unfortunately, something that they leave incomplete are modem scripts--which tell your Mac how to speak with your phone as a wireless modem.  That's okay, because Ross Barkman has
provided all of those scripts for us.  You might consider droping him a few bucks for all the work he's done here:

Here's a list of the current phones and their respective downloads (they're archived as ".sit" so you'll need StuffitExpander):

Motorola: GSM (for Cingular and T Mobile), EDGE (Cingular EDGE phones, like the V551), CDMA (Verizon), if all else fails (for non-GSM).

Sony Ericsson: GSM (for Cingular and T Mobile), EDGE (Cingular EDGE phones--not sure any are currently offered in the US).

Nokia: GSM (for Cingular and T Mobile), EDGE (Cingular EDGE phones, like the 6620), CDMA (Verizon).

Siemens: GSM (for Cingular and T Mobile).

After downloading and unzipping the appropriate modem script to your Desktop, in the folder, you will find a "Readme" file and some scripts.  Copy and Paste the modem scripts to your /Library/Modem Scripts folder.  This is not the Library folder in your User directory, but the one in the Library folder you should see if you click on your hard drive (by default, it's named "Macintosh HD") on your Desktop.   Make note of the names of the scripts because you'll need to select them from a list later.  You should also read the Readme for any potential gotchas specific to your phone and modem script that this howto might not cover.

Setup your Phone via Bluetooth:

If you haven't already setup your phone with your Mac, follow the instructions I provided in my first review, here.

Add your Network Provider's Settings:

This is one of the trickier parts of the setup, and unfortunately it might require some trial and error.  To setup your phone to get on the Internet, we've got to tell the Mac that we're going to use the phone as a network device.  So, where we need to go is to Network in your Mac's System Preferences.  From there, in the "Show" drop down, select "Bluetooth."  If you don't see "Bluetooth listed, make it active by selecting "Network Port Configurations" in the drop down list and make sure that "Bluetooth" is checked.

In the Bluetooth network preferences, select the last tab, labeled "Bluetooth Modem," and then from the Modem drop down list, find the modem script that you installed above.  For the moment, keep all the boxes checked right now (we may have to play with those settings if the connection doesn't work).  This should also add a new icon  of a phone receiver in your task bar. 

Now click on the first tab, labelled "PPP."  Here's where we're going to have to add some settings.

For other Network Providers outside the US, click here for the list that Ross has compiled. For T Mobile subscribers, the account name and password of "guest" is used because apparently, T Mobile doesn't require anything specific, they just require something.

Additionally, Cingular and T Mobile offer upgraded data services that require subscriptions.  The settings for those services are listed in the second and third rows.  Cingular offers separate settings for data connections with faster data compression, which does not always work.  T Mobile provides separate settings for those using the connections for a VPN connection.

Now click on the "PPP Options..." button and uncheck "Send PPP echo packets," and click Okay.  Make sure "Save Password" is checked.

Whew, got all that?  If so, I think we're ready to fire it up! Click on the "Dial Now" button and up should pop up the Internet Connect window, showing the settings you just entered.  Click on "Connect" to start connecting.  Your phone might beep, asking you if you want to allow your Mac to connect to it; select yes.

Now, you should see the connected screen, which shows you the through-put monitor, hopefully showing some data connections.  Launch your favorite browser and see if you get a connection.  Remember, this is not going to be as fast as a WiFi or ethernet connection, but if you're on the road and need to get onto the 'net, this is one way to get online.

Follow Up...

I'm hope to provide details on how to connect your Mac to your Verizon, Sprint, and Nextel cell phones and to update the page soon.  For any questions or comments, please click the button below.