I'm new to looking at the web as a business, but have owned businesses in the past. It seems parterships are dificult. The only train of thought that I have had for online parterships and keeping the finacial/tax aspect
extremely simple are for a site that earns its income strictly on ad revenue. In that case perhaps an agreement can be reached like "my one ad goes here, your one ad goes there". That way you both keep on with your seperate set of books. The cost of hosting would be part of the negotiation in regards to who's ad goes where.
As far as making sure a partner does his/her share of work, that's dificult. My business experience is a lot of things take longer than people's impresion, and it's somewhat hard to guage who spent how long doing what. I have no ideas on the labor portion, except that 2 good minds can sometimes be equal to more than the sum of their parts. I have to know someone very well, or maybe catch them being honest when they thought no one was looking.
Just my .02.
Goodluck,
-John