I have one of the first Cingular branded v3xx phones that was released when they first hit the market. Although it came pre-stocked with shortcuts to Cingular stores, Cingular Music downloads, and had things like the IMAP email client disabled by default, it was still relatively easy to hack. I was able to use P2kCommander over a USB cable to enable cool features like allowing unsigned java applications to access the internet , and the IMAP email client.  

To use P2kCommander, you had to go into your Settings->Connection->USB Settings and make sure that the default connection type was set to Data Connection (as opposed to Memory Card or USB Printing). If you couldn't set it to Data Connection, you can't use P2kCommander. (Or use the phone as a modem via a USB cable.)

A reader recently wrote in that his new Motorola V3xx phone from ATT / Cingular has been "extra crippled" such that the USB Data Connection is not even an option! His phone has the following software versions: 

Boot loader: 08.26
SW VER: R374_G_0E.41.C3R_B

My phone is obviously "older", with the following information:
Boot loader: 03.02
SW Ver: R26111LD_U_96.71.95R
 and did not have the same problems. (To find this information quickly, turn the phone off. Hold the * and # keys down, and tap the Power (red) button to start the phone. The phone will start up in bootloader mode, showing the bootloader and software version currently installed.)

These new V3xx's probably don't suffer from the sim card lock problem , but since you can not use P2kCommander to change their settings, the only option you have is to get it unlocked and then flash it with a Motorola generic firmware.