There is no "preference" stated here.Without compelling evidence otherwise, I tend to subscribe to the theory of revealed preferences - our actions reveal what we really want.
But you are logically correct, there are some possibilities, e.g., the poor would prefer to have a job but irrationally don't look for one.
I somehow doubt that's because people "prefer" to remain poor.
You are looking at the wrong choice set. The choice set is not [ poor, middle class]. The choice set is [ (poor, leisure), (middle class, hard work) ]. The theory I'm pushing is that for some people, utility(leisure)-utility(hard work) > utility(middle class) - utility(poor). See my blog post for more details.
Also, this is not caused by the current recession. The numbers have been similar since 1996 (the year of the earliest report I can find with a quick google search).
http://www.bls.gov/cps/cpswp96.htm
http://www.bls.gov/cps/cpswp2002.pdf
http://www.bls.gov/cps/cpswp2006.pdf
(Tweak URLs to get data for intermediate years.)