In posts you've downloaded or a post you're composing?
In the default mail program on my 2.2 Desire, text selection and copy works with downloaded mails just by a long press on the text until the text selection markers appear.
In the mail composition window, a long press on the text brings up a dialog which includes the option to Select Text. I can then drag the insertion point to a new position to select text, or use the trackball to do the same. In this case, the current insertion point marks the start or end of the selected text, depending on the direction in which I drag.