If you are lucky enough to own a Nikon 14-24 f2.8 you may be interested in making one of these http://reedingramweir.wordpress.com/2010/04/07/diy-filter-holder-for-nikons-af-s-14-24mm-%c6%922-8-g-lens/
, I have the parts arriving today complete with Cokin ND4 filter.
The down side is that the darkest Cokin do in the X-Pro series is ND4, but you can double them up to get even slower speeds, I have one on order today.
All parts including ND4 filter £152. The Lee SW150 costs £300 so quite a saving for very little effort.
PS thanks to Reed Ingram for the Excelent idea and parts list
PPS there is complete coverage at all 14-24mm so the entire zoom range is covered by the filter on my D3s (FX)