Came across the James Kroeger's web site (http://www.taxwisdom.org). He lays out economic justifications for maintaining and extending progressive income taxes. It looks good to me: lots of simplified ceteris paribus reasoning, like economic models always have, but an interesting argument.
However, this is outside my field, and I can't help but wonder if his arguments stand up to scrutiny. To the econo-wonks on the list (you know who you are), take a look at the web page. Any egregious howlers there?
Miles