Research has shown that the majority of people classify themselves as middle class (I got my undergrad in Sociology, but I don't remember the reference). Middle class is the "norm" and everyone wants to be normal. Whether that is justified or not cannot be proven, because there are too many variables and not everyone agrees.
The New York Times did an article a while back (2005...or something) to put people into classes, but that's just the determination of one publication. Though I'm sure they used Census data to determine which things fit into which categories.
Here's a link to a graph that will generate a class for you if you're curious:
http://www.nytimes.com/packages/html/national/20050515_CLASS_GRAPHIC/index_01.html
It also has some results from some surveys done about class.
By it's determination, we have no class due to occupation, because military officer and stay at home mom are not included on the lists (we would probably be somewhere in the middle considering the nature of my husband's job, but who knows?)
Our Educational class is considered to be in the Top 5th, because we both have Bachelor's Degrees.
We're somewhere between Middle and Upper Middle based on our income (depending on if you include our housing allowance)
Our Wealth would put us in the lower class, because we're still in debt (student loans) thus we haven't had the chance to build up our savings or buy our own home.
In general, class tends to be based on perception, especially the occupation category. The other parts are a bit more quantitative, but if you were to tell someone that they are lower class, they would likely argue with you, even if the numbers told them otherwise. Nobody wants to be lower class, and many Americans act as if they are middle class even if they don't have the funds to back it up. Oddly enough, there are some people who are upper class that still classify themselves as middle class. The rationale behind that confuses me more than the lower saying they're middle, but I suppose even many of the very wealthy wish to be considered normal.