I haven't been able to get it to work everytime, but like in text's, just touch and hold at the beginning or end of where you would like to copy. Then, after a moment, a box will open up with options to select text, select all text, and a couple of others. If you only want to select a portion of it, just select, select text, and and then with only one finger at a time, move the highlight field to cover what you want to select. When you're done highliting, press and hold again, and select copy. Then go to where ever you would like to paste it, and press and hold the screen again, and select paste, and it will insert the text that you copied. It doesn't work the best, and like I said too, not in all areas of the phone.
That may work with the GMail client but it doesn't work with the built-in e-mail client or the Calendar app. Are there any plans to fix the total lack of useable copy & paste functions and phone number auto detect (which should launch the dialer with the number loaded and ready to hit send) like I can do with my BB or like I could with my Moto Droid?