I just today called TuYo Mobile (1-877-GET-TUYO) to claim not one but two totally free SIM cards (each has $10 worth of credit on it) for their service...nice...
1) TuYo runs on T-Mobile's network, including roaming (I caught my Nokia 3120 on Cellular One here!)
2) TuYo's plans are nice and inexpensive for voice, with long expirations (10 cents a minute plus 10 cents to connect for low-volume users; cards as low as $10 expire in 90 days)
3) Text messages are 5 cents to send and 2 cents to receive (so you can receive twice as many texts as you send and it'll still equal out to less than 10 cents per message to send\free to receive, which is getting rarer anyway); MMS is 25 cents to send and a mere 5 cents to receive
4) The customer service rep I talked to said that normal, priced-per-day-or-per-meg internet is in the works! (This is a biggie for me because I now have a Palm TX which I can use in conjunction with my Bluetooth-enabled Siemens s56 to access internet anywhere, provided I have service that'll let me do it)
So I'd say get one or two of these SIMs while you can! It's a pretty nice deal.