[brklyn]

No need for a custom rom, I have a Diamond and am on stock sprint firmware going fine with phone as modem.

You need to change a few registry keys if you don't pay for the phone as modem plan. This will let you tether for free.

1. Edit the following registry entries.
[HKEY_LOCAL_MACHINE\Comm\InternetSharing]
Extension="" Delete the "isext.dll" entry

[HKEY_LOCAL_MACHINE\Comm\InternetSharing\settings]
ForceCellConnection="Sprint PCS"

2. Start the Internet Sharing application.
3. Hit Connect In the Internet Sharing application.
4. Connect the Diamond to the Laptop/PC via USB or Bluetooth