If you have an iPod Touch, then there’s a simple way to send text messages without having an iPhone. There’s a UK-based service called ipTexter that allows you to rent a virtual phone number that receives text messages over the web. You then have an application that is capable of reading these messages for you. The service costs £7.99 a month, and then you buy text message bundles.
It’s a little expensive in my opinion, and if you look really hard, you should be able to find some free or cheaper email or web based SMS services. This company seems to be using the application as an interface to the service rather than using an email or the web.