I did it to my WF860G by removing the two screws at the back and pulling hard from the back. The lid makes a terrible noise as if it's breaking, but it just snapped out.
Don't lift it up more than 1cm or so, or you might break the plastic catches.
You push it in the same way to put it back on. It goes back a lot easier.
However, if you break it, don't blame me!
uk nod, March 2007