when internet connection sharing is activated the device disconnects, then switches to your
username@modem.sprintpcs.com then tries to connect again. if you do not have PAM BASIC on your account your device will not authenticate on the server with that username (error 67).
If your device is running WM5 (like the telus devices or pre-production sprint devices) it should not switch usernames. It may be possible to replace the .dlls and exe for the app on WM6 to negate this problem... this is untested, just my speculation.
Hope this helps.