Julian, considering you have the 17mm + 24mm TSE's, what does the Nikon offer?
Well, Lee filters has just released/produced a filter holder for the Nikon 14-24. This is something I've been waiting for a while now(mine's preordered @ 2Filter.com). Considering that I do have the 17 and 24 TS-Es, the 17 isn't capable of using filters at this time. This is a big concern for me bcoz' 24mm will not be wide enough for certain situations. Lee has announced that they will be adapting for lenses that have bulbous elements in 2011. Do I know which lenses are going to be adapted? Not sure. But, I have this feeling that Lee will make adapters for lenses such as the 17 TS, 14 L, Sigma 12-24, Tokina 16-28, etc. I am aware of my overlap with wide angle lenses but, I'm UWA angle fanatic.
Here's a link from Lee filters USA. Read the third paragraph down.