DoubleClick posted some data that the NYTimes picked up yesterday, suggesting that fewer email recipients are clicking on emails (spam filters, or blocking the tracking images may be contributors), but of those that are clicking, there's better conversion.
From the DoubleClick press release:
“We have seen open rates declining for several quarters and we knew that that it was in part as a result of image blocking by ISPs and Outlook. What this in-depth review shows is that it isn’t one individual factor that is putting pressure on open rates but a combination of forces, and with this insight we can help marketers to better analyze and target their e-mails to various list segments,” said Kevin Mabley, Vice President of Account Management and Strategic Services for DoubleClick E-mail Solutions.
Another contributor might be that their tracking cookie (from DARTmail) is set via a third party cookie, and there are more third party cookies getting blocked.
Also, the anti-spyware tools might be identifying and blocking the doubleclick tracking requests.