I hope i'm not going to stir up the nest again, but I have been thinking of a similar idea for a while now.  To 'rent' my small rendercow farm out to people cheaply, as i'm not really using it a lot at the moment.  I have 23ghz of rendering power (2 core duos, a quad and an AMD64). 

If anyones interested i'll say 1c (aud) per gigahert hour.  So effectively you would be 'renting' my 23ghz renderfarm for 23 cents an hour, which is, if you look around, incredibly cheap.  However,  this is only for a trial run.

Oh one other thing.  If you feel that you have waited too long for the render, you get it free.  Again, only for the trial run.

If your interested send me an email: paul at chromesphere dot com for more information.

CS