I wasn't quite sure what type of messages you wanted to send but there are several apps in the App Store that will let you send text messages to a mobile phone - usually for free. This is done indirectly via the developer's website so you don't need a 3G iPad to be able to use these, though you do need an Internet connection. The service is usually funded by advertising - i.e the text message will contain, in addition to your message, an advertisement. If that's acceptable to you, then there's no problem and several companies offer that service.
Tim