To switch from worksheet to data grid I use CTRL+TAB TAB CTRL+TAB TAB. It looks difficult but once you make a habit out of it, it goes quite fast. The first CTRL+TAB gets the focus from worksheet to the SQL button on the data grid. The TAB then gets the focus to the line linumber column. The next CTRL+TAB then gets the focus somewhere (can't see where) and the next TAB finally gets the focus to the first data cell in the drid, where you can finally navigate with cursors keys (and do a CTRL+C to copy the value to the clipboard). Don't ask me how much time I put into finding this...
To navigate back to the worksheet press the CTRL+TAB TAB combo again.
I use SQLDeveloper on Linux (Gnome 2 desktop - Centos, RHEL and Oracle Linux have this). On other operating systems the key combinations may vary.
Hope this helps