Steve, very good point. This is business and business is about money. The potential JV partner will need to know he is making a good investment, so if you don't have inherent credibility in the industry, you better have a good sales pitch based on numbers...reliable numbers. Like you said, conversion data is key.
Another point to add is that you will likely want to approach these potential JV partners well before you speak to them about any JV inquiry. Build a relationship, build rapport, perhaps offer your services for free. If they have a blog, tell them you've been following it and would like to do a few guest articles that their visitor base would enjoy. If they have a forum, perhaps ask to be an unpaid moderator (of course be a member first). If you are good at copywriting, tell them you can write up some split tests for their next email. Do this as a one way gesture. Build the relationship and over time if you work well with the potential JV, things will develop further and a successful JV partnership may come out of it. If things don't work out from a JV standpoint, you at least made a friend in the industry...sometimes those types of relationships reap dividends years down the road. Don't burn bridges.
Okay, I am rambling here but it's mostly because I feel so strongly about giving giving giving before even thinking about getting. That is exactly how I forged ahead in this IM game and it works.