To me it seems that cooperative mining is a tough task, because the honnesty of participants has to be checked.  What's preventing someone to run a modified version of the client, that would just keep generated bitcoin for himself, while receiving bitcoins from others ?

I wonder if it wouldn't be simpler to just rent processing power.  For instance by renting a ssh access to each machine.

The price could be directly proportionnal to the total number of computed hashes.