If you really ONLY want to text message and don't need the cell phone or the internet part...I recommend the Motorola T900 2-Way Text Pager. I have had mine for several months now and have been very happy with it's service. It's small, cute (us girls like cute things...lol), and can be clipped to a belt or pocket, etc...and has a strong vibrate mode to let you know when you have a message.
I recently travelled from the Atlanta, Georgia area to Biloxi, Mississippi and I still got all my messages the whole weekend that I was away. It also gives me news headlines, weather, etc...(I really didn't need that, but it came with it)
About the only drawback I have found (and there may be a workaround to this and if someone knows of one, PLEASE let me know) is that people have to be online or have a text-messaging enabled phone to send you messages. In other words, I haven't found a relay service where people can call and give an operator a message and have them send it to your text pager.
Anyway, the T900 is pretty cheap, and in some cases and depending on what plan/carrier you use...it can be free, and I pay about $20 a month (US) and get 1000 messages per month...which is more than enough for me.