In reply to gareth uglow:
One for the FAQ, I think...
The method I've used successfully for holes of this size for a number of waterproof fabrics (GTX3, TPC, etc) is to use iron-on seam seal tape, available for about £1/m from Point North or Pennine Outdoor. This is quite effective at closing tears, and seems to be pretty durable. It's also worked on fairly long, linear tears.
It can be ironed onto the inner scrim face of 3-layer fabrics, but must be ironed onto the outer face of 2-layer Gore-tex, as it won't readily stick to the PFTE Gore-tex membrane.