It fits the trend. Union membership is steady declining since 1983 - it dropped another 0.4 percentage point in 2004 http://www.bls.gov/news.release/union2.nr0.htm.
Of course, we can speculate about possible reasons of that decline - hostile legal environment, capitalist propaganda, intimidation, etc. My favorite explanation is that there is little pride and status related to being "working class" - everybody wants to be a "professional" - a professional truck driver, a professional janitor, a professional sales clerk aka "associate." Union membership, otoh, involves identification with the working class and its ideals - and fee people want to do that.
I had a nice chat with a conductor on an Amtrak train in Pennsylvania not long ago - he was a rare example of someone with a "working class identity" - proud of his job and its status, staunchly pro-union. However, he was aware of the fact that he was a dying species. According to him, his son who was a truck driver and his son's buddies treated their jobs mainly as a cash cow, trying to get as much money as possible by any means. They refused unionization because they did not want pay the union dues and saw unions as an obstacle in their money making.
Yet another sad example how reactionary this country is becoming.
Wojtek