The following was posted in Android Lounge as a great solution to this problem, by pbf98:
"also depends on keyboard with sms, i know if you are using swype for a keyboard you can press the number key at the bottom, and then long press the same button that turns to abc.. it then brings up the text selection keyboard, with that you can select all or one letter.. cut, copy, and paste.. you can go from "home" to "end".. there is also page up and page down.
Something that is useful to know if you use swype "
