The traveler's check provision comes and goes--but it is back on the web site. I prefer this method as it avoids having to carry around a lot of cash and I don't need to worry about finding a working ATM at the end of the trip to get the tips together (you give [in envelopes provided] to the guides at the farewell dinner).
I called ABD today on an unrelated matter but happened to ask about tips---and she noted that the guides are very resourceful, so local currency, US dollars or traveler's checks are fine. I wouldn't stray from those three options, though!
(The booklet you'll get has a recommended range for tips per guide per individual, and usually give an example of a family of four and what the range would be--given our great experiences with our guides we alway pay tips at the top of the range--sometimes we will even toss in additional currency if they did something above and beyond!)