I used an M900A when they first arrived on the scene, and noted that when they got even the slightest amount of moisture inside, the LED lamps would "glow". The light still worked, but if you were in an environment where compromise could be an issue, the LED's glowing, did not give a "warm fuzzy".
I chose to "waterproof" mine by using RTV and sealing up any exposed wiring and any and all gaps I could find in the housing. This worked against large splashes of water. Unless you plan on swimming with it, you may prefer the cheaper method. This method, however, WILL VOID YOUR WARRANTY.
I have since (two weeks ago) asked Sure-fire about a waterproof model, and they did in fact tell me that the "N" model was in production.
I can only imagine the hike in the prices though.