You should be able to find a rigid fork that keeps your geometry close to a 100mm suspension fork. I recently bought a rigid fork and noticed some companies will describe it as either suspension corrected to 80 or 100 .... Or give you the axle to crown numbers...468mm is approx 80mm suspension...Not positive what a 100mm corrected exactly is. Also, if your trying to calculate the numbers yourself, don't forget to factor in the sag on a suspension fork.

The niner carbon fork is awesome! I really wanted one but could never justify the price. Whiskey parts, ritchey, carver, and a few others make carbon rigid forks for a little less. I ended up getting a chrmly salsa take off for 30 bucks. A little heavy but does the trick.

 

I believe most rigid forks that are "corrected" for 100mm travel on a 29er have an axle to crown length of around 470mm.

As DRZ noted, the effective axle to crown length of a suspension fork decreases when you sit on the bike due to sag. The amount of sag varies with the particular fork, rider weight, and how you have it set-up (preload, etc.), but most seem to squat down ~20mm.

The Niner fork should be fine.

 

470mm a-c might make your bike better for smoother terrain

 

Just for the record, my Reba RLT 100 mm travel fork on my Stumpy EVO ht is exactly 1/2" longer than the 470 mm whiskey # 7 on my rigid 29er SS.