Is it registration to be able to submit an order, or to provide leads for information, or for a membership? That can make a difference, although generally studies have shown that the simpler the better.
BTW, here's a classic thread on site conversion with a number of good tips based on people's experience and testing:
Msg#: 3901486 posted 9:02 am on May 6, 2009 (gmt 0)
Yes, I know that but I have to convince my CEO that more mandatory input fields is a bad idea. Therefore am I looking for a report or studie that clearly shows how the abandon rate increases with more mandatory input fields.