As "waow" says, he is pretty much right on the money. I am however unsure
how this would work on a Android phone without a physical QWERTY keyboard.
I own a G1 and I had this problem the other night when I needed to copy the details of an E-Mail and send them via. SMS. I am running the current non-rooted 1.6 "Donut" build. Holding down on the screen over text did nothing as well as hitting the menu button yielded no options for selecting or copying text.
By holding down the shift key, it will bring up a cursor that will allow you to hover over text and you then click the trackball down once and release and then use the ball to scroll down (While still holding the Shift key) and highlight the content you want selected and then click the trackball again and it will then say "copied to clipboard". It's not intuitive, but it does work for me.
I suppose, for non-keyboard phones, you would have to log in to your Gmail account through the browser and then select the content to copy that way.
I hope this helps!