I believe he did work with ACORN and other activist groups.
But his other associations will have your spider-senses tingling. Check these out - some of it you may be aware of.
http://www.pittsburghlive.com/x/pittsburghtrib/opinion/columnists/datelinedc/s_488184.html
http://www.discoverthenetworks.org/individualProfile.asp?indid=1511
http://www.americanthinker.com/2008/02/barack_obama_running_on_his_re.html
Dang. I bet he’s got the vote fraud thing down pat!