There's a great post over at Blue Sky Factory addressing the fact that unlike the controversial behavioral targeting feature which Google has implemented, as email marketers we are in a wonderful position to use behavioral tracking – as we have permission from our subscribers.

As Nikki says :'Email marketing is one of the best online marketing mediums to offer
permission-based behavioral tracking for online users.   Bottom line: it offers
marketers a way of establishing relationships, cross-pollinating social media
networks and RSS feeds, and it builds your client base organically. Email
marketing enables marketers to deliver campaigns specific to the interests and
behaviors of their recipients, while giving measurable results to easily track
your ROI.'

Read the full post