Stackoverflow had an interesting conversation about this email confirmation & captchas.
I think the jist of it can be summed up like this:
- captchas help protect against automated signups
- email confirmation helps protect against impersonation
I think though, the real question is - how to increase conversions.
Take a look at this article in a list apart that talks about how signup forms must die. A bit harsh, but the core idea is to engage the prospect, capture as little info as possible, then automate the signup.
The fewer the fields the better - and the later in the process, the better.
Here is another article - smashing magazine - about web form design best practices.
(click to page 1 of the article - talks about the methodology and players)
To get to the specifics of your question, you could potentially simplify your signup to a single email field (password could be auto-generated, username could be added in profile) - if you believe that would make a difference.
Some may say, jeesh, its just a simple field, why the bother? Take a look at this article from user interface engineering entitled the $300 million button. Takeaway: people resist giving information away and abandon signups if presented at the wrong time. Usability testing can confirm this - there are crowdsourced service providers (usertesting.com, for example) that can provide valuable input for under $100 (i think $29 / user x 3).
My favorite line from the article - usability reviewer: "I'm not here to enter into a relationship. I just want to buy something."