The folks at Pivotal Veracity have written a report about how ISPs are changing their spam filters and how legitimate email marketers are impacted by this. You can download it here.

Key take-aways:

• Domain-based Reputation is here!
  A number of top ISPs including Yahoo & AOL are moving to augment IP-based reputation systems with portable Domain-based reputation systems for those mailers using DK/DKIM authentication.  This is a hugely important development and one that will be welcomed by legitimate mailers.  This means, ISPs will “attach” (compute) your Spam Complaint Rate, Unknown User Rate, and Spam Trap Rates to your Domain (this will be the domain you are authenticating which for most mailers will be the friendly from domain)  in addition to your IP.   Switching IP addresses? With domain-based reputation ..you get to keep (for good or bad) your reputation.
• Authenticate with DK/DKIM
  With the exception of Hotmail which is still sticking to its proprietary “Sender ID” authentication model, the authentication method of choice by all the other leading ISPs is DK/DKIM.  If you are not already ..authenticate !  You need to authenticate in order to take advantage of domain-based reputation and other ISP services such as Yahoo’s feedback loop.