If we assume that the 2 cases per 100,000 people is a valid number then the number of cases would have to increase. The population in the U.S. is increasing by 1 person every 13 seconds. For the sake of argument let's also say that the 5,000 cases per year in the U.S. was correct as of January 1, 2005.
31,536,000 seconds in a year * 3 years = 94,608,000 seconds
94,608,000 seconds / 13 seconds per person gained = about 7,000,000 people (increase in U.S. population in last 3 years)
7,000,000 people / 50,000 (2 in every 100,000) = 140 additional cases of ALS in the last 3 years in the U.S.
Meaning that as of Jan 2008 there would have been 5140 instead of 5000. Given the fact that there is no centralized tracking and even the ALSA doesn't touch everyone diagnosed you'd never see the change.