Looking under Options>Advanced Options>Host Routing Table> there's a bunch of T-Mobile USA [310XX030] entries. My phone always seems to be on the same one whenever I check.

But there's one Wi-Fi[60] showing. I tried to register for it to see and it just said "message sent" or something and it's still showing bold on the one I am registered for.

What is this for?

The entries other than the WiFi are MNC/MCC codes; what that means is they are the ID numbers of the various networks throughout the country. 'Host Routing Table' is RIM's homegrown lingo for the collection of preferred networks that T-Mobile wants you to use (who they have roaming partners with, etc.) - it works in combination with a similar list that's kept on your SIM chip.

The WiFi entry you see is what I presume is the route your data service takes when you enable WiFi on the device, connecting to RIM's servers for your email. It's basically the same as the typical MNC/MCC tower codes in the logistical sense, it provides your device the path to take to connect to the upstream network when WiFi is turned on. When you turn on WiFi the device negotiates with RIM's servers to manage your email over the wireless network instead of the carrier (aka 'Mobile Network').