Considering the implications (especially security concerns), copying to/from clipboard is not something that is quite allowed / easy.
Those questions / answsers might help you, about that :
In each case, it seems that the solution is to use some little .swf file -- which means working with flash (maybe some old version of flash, actually)...
For instance, you could take a look at the Zero Clipboard library (yeap, flash...) :
The Zero Clipboard library provides an
  easy way to copy text to the clipboard
  using an invisible Adobe Flash movie,
  and a JavaScript interface. The "Zero"
  signifies that the library is
  invisible and the user interface is
  left entirely up to you.
This library is fully compatible with
  Flash Player 10, which requires that
  the clipboard copy operation be
  initiated by a user click event inside
  the Flash movie. This is achieved by
  automatically floating the invisible
  movie on top of a DOM element of your
  choice.
In pure Javascript, it seems to be possible in Internet Explorer, but not in other browsers...