I use 26 x 1.5-1.9 tubes in my 29er mtb tires (29 x 2.1 to be exact). Takes a bit more effort to mount the tires, but the tubes work just fine once mounted. Saves me about 100g per wheel over a "29er" tube.

Feel the area of the tire at the valve area. Make sure that it doesn't feel like the tube is necked down in that area, i.e. the tire shouldn't feel any narrower there.