Letting the contributors decide would unbalance the workload; they
would always choose "popular" projects like cancer or AIDS, which you
mentioned.
Maybe a Calendar integration for just specifying whether or not the
client could run would be cool, for example only between 12AM and 8AM
or something.
I use folding at home and i have a project thats is going to take at
least 5 days non stop computing (it has 2 left). If i switched
projects every day the current project could take weeks to finsh. If
the proejects were made even smaller though then this would be
possible. (some of them take about 24 hours of nonstop computing) The
general idea is to have a project completed in about 1 week.