thanks for the replies.
I'm out of contract, so my options are open. I have an HTC Sensation, and from what I've read, it seems to work on the prepaid services. My wife has my HTC Shadow hand-me-down, so I'll need to get another phone for her. I'm considering the Samsung Galaxy Exhibit.
The $30 plan seems like it can only be completed online or at Walmart, and it looks like a headache to get the plan activated and port an existing postpaid phone number, reading the T-Mobile forums. I'm preparing to buy the SIM cards through the T-mobile store.