It's something one of my professors (PhD from the Kennedy School of Government) is currently studying. I'm sure I've got some data on it somewhere but it's not like I sit on it.
I don't dispute Rudey's stats but if you look at overall household income in the Democratic party (by which I mean registered Dems) it's been actively rising and is also brought down my students.
Another thought I have, which is just conjecture, is that Democrats are much more likely to live in an urban area and thus may not own homes although they are wealthy.