User agents of clients that provide support for the RFC 3229 "feed" instance manipulation method:
Bloglines/2.0 (http://www.bloglines.com; 8 subscribers)
This data is gathered from looking at the Apache server logs for today and yesterday. Bob Wyman is tracking implementations of RFC3229 with "feed".
Note: this list may include browsers who merely "stumbled upon" the correct URLs of the history files
Sam: I'm now triggering my delta-generation code on the existence of A-IM: feed, and I've included a little snippet of code that logs the request header and the delta delivered... so I should be able to compile a list of compatible clients without any false positives.
Whether or not I'm doing any of this appropriately is up for debate, of course. :)