[qbunnie]

There is a lot of good threads about this, so if you get confused, you should look around a bit.
Here is a basic rundown as I understand it:
While it seems that data is data, when you connect to Sprint's data services, you actually login with a username and password --> this determines your "NAI". For programs like pocket internet explorer and such, you use your normal power vision data service. Since Phone as Modem is a separate package (it used to be $40/mo...), to use Sprint's PAM services, you would login using a separate PAM specific username and password. These are all built in to the phone, so you don't really type them in ever.
Normally, when you fire up ICS, the stock settings login using your NAI for the PAM plan. If you don't have a PAM plan, you get an error67 (i think). The first workaround for this was to login to data services using PocketIE which uses your powervision credentials and then open ICS. The active data connection just transferred over. You might not have to do this if you are using DirectPush, since your data is always on. However, you might still get a Error67 (if the data connection becomes idle), which can be quite annoying as even a normal soft reset wouldn't always restore even normal cell phone service. Make sure you have a backup of your phone (good programs are Sprite (not Sprint) and SPB Backup) Then it was found that deleting a file called something like rllphone.dll would no longer try to login using PAM credentials, thus you could use ICS directly without a problem. However, this is up to debate, and I can't be 100% sure that is what's happening. Anyhow, if you are willing to spend $34, PDANet by Junefabrics is a great program with good support. It even allows tethering while keeping a voice connection active. It masks your data usage as normal phone browsing, thus forcing the data to be your powervision NAI and not your PAM NAI. Of course there are cracked versions, but that is not supported, and IMO, this is one of those programs that is worth the money.