I'm posting this thread from my Boost Mobile Motorola i455 phone and my Mac Mini.
I've been following Purdoom's excellent
Boost Mobile thread but the discussion is for Windows users. I wanted to access the net on my Mac using Boost Mobile. Why?
Boost mobile is pay as you go. You put money in it and you're good until the money runs out or 3 months. You can add data access, giving you mobile internet for just 35 cents a day or about $10 a month, no limitations. End of story. It's slow, but all I really want to use it for is to broadcast my car's position using the built in GPS in the phone and the
mologog application and perhaps use it to refresh the Google Earth cache when it runs out. It appears to be fast enough for that.
I only have a cell phone for emergencies so I really could care less what the per minute rate is. I'm not going to burn it up talking on it anyhow. After poring through a lot of posts, I got it working. Here's how.
1. Purchase the Boost Mobile Motorola i455.
Approximate cost $60. It comes with $10 of free airtime. The i455 is an iDen phone, which means it goes faster in urban areas with that type of service (it's on Nextel's network). Too bad I live in the sticks and won't benefit from that very often.
2. Purchase a USB data/charger cable for the phone.

Approximate cost $20. The first one I bought didn't fit even though the ad said it would. The proper model number is NNTN5405. Make sure you get the right cable. Thanks to Thanatos for the link. I found a Buy It Now on Ebay and got the right one.
3. Activate the phone and make sure you add the data service to the plan.
4. You'll need to do two things to make the modem active, then you'll have to set it up. First, get a modem script for the iDen phone. I found my script here:
OSX Hacks. Follow the instructions to put it in the Library/Modem Script folder.
Restart the computer then plug the phone into the cable and the cable into the computer.
5. Second, you have to tell OSX to activate the new connection. You do this by going into the System Preferences-->Network panel. You should see this

.
Now you want to choose the "Network Port Configurations" in the drop down list next to "Show:" You'll notice that the phone shows up with a blank name.
That's your phone but don't worry about renaming it just now.
6. Now that OS X knows about the phone, you need to set up a location that allows you to use it in the car. I found that the easiest way to do this was to use the Assistant.
A. Click on "Assist me..." at the bottom of the window and you'll see a window that asks you if you need assistance. [insert image You do. Click on the "Assistant..." button.
You'll see a window that asks you to name a new location. I chose "Motorola i455" as my location name. Click "continue".
B. You'll be asked how you connect to the internet.

Choose "I use a telephone modem to dial my ISP". Click "Continue".
C. The assistant requires you to enter stuff that isn't necessary. All you really need is the ISP Phone Number, but the assistant requires you to enter an account name and password. Since they don't matter, I chose "Mobile" as my account name and used "some" as my password. You can use whatever you want. The only thing that really matters is that you use S=2#777 (I've also heard that S=2 also works) as the ISP Phone number.
Click "Continue".
D. Now you tell the Mac which modem script to use. From the drop down, find "Motorola i85 iDen". It it isn't there, you have forgotten to put the modem script in the Library-->Modem Scripts folder or you have put it in the wrong place. Also, check "Ignore dial tone". Click "Continue"

E. The assistant will now show you a confirmation screen and give you an opportunity to try to connect to the internet. Go ahead and do it. If everything is working properly, Internet Connect will open up and you'll see the following as it connects to the internet.

The assistant should confirm for you that you are connected to the net under the location name that you picked.

You can now surf the net, pick up your email, run Google Maps, etc. Whatever you can do with the net, as long as you are willing to do it S-L-O-W-L-Y (I get 2400 baud here in the sticks).
7. When you are not using your modem, you can switch back to your old configuration by choosing Apple-->Location and selecting the proper location. In my case, you can see that "Automatic" is my out of the car configuration.
When you want to use the modem again, switch back to the Motorola i455 location.
8. One more thing you want to do before you are finished, is to set the modem up so its ready for the car. Go back to "System Preferences-->Network and select that blank line in the "Show:" drop down that represents your phone. It should look like this:
9. Select the PPP tab and then the "PPP Options" button and check "Connect automatically when needed, uncheck "Disconnect if idle for 10 minutes" then click "OK"

Close the control panel and make sure you answer "Apply" when it asks you to save the changes.
That's it! You're now online with the net from the car for 35 cents a day! Check email, use iChat, surf, just really slowly. Too slow for you? Try Verizon's EVDO for $70 a month. It's much faster.
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Update: I have installed Mologogo on the Boost Mobile phone. Now you can see where the iBug is at all times. Go
here to see where it is right now!