I wrote a post on the FeedBurner forums yesterday asking how to track RSS subscriptions as conversions.
The reply from Dick Costolo confirmed that:
This is on our roadmap but not yet possible. We have four or five more things we want to add to site stats to make it “whole” and then we will begin to bridge this blind spot between site visits, subscriptions, and conversions from site visitors to subscribers and the crossover therein.
This probably takes us a couple/few more months as this also implies that we need to stop having two separate stats dashboards and create a common content-centric view of stats in which you see your subscription conversions and crossover traffic (eg, “28% of commenters are subscribers”). We are very excited about this, but as always, the good stuff takes time to build :)”
This sounds like an excellent feature and one which I would definitely be keen on using to track the type of traffic and keyword phrases which convert into RSS subscriptions.