It seems that some providers allow inbound TCP/IP connections to phones, while others do not. My own Verizon Droid seems to have no issues here. I'm seeing complaints with WebSharing/WebSharing Lite with T-Mobile users who claim they're unable to use the cellular sharing feature. It also seems to possibly be at a more fine-grained level than just the provider. E.g., some regions allow it, while others do not. Would greatly appreciate anyone on T-Mobile letting me know if they are able to receive inbound connections. You can download WebSharing Lite, turn off Wi-Fi, and then start WebSharing to determine this empirically.