Dustin any site with a large email subscriber list has a very easy solution to have a <a href="http://andybeard.eu/2007/03/do-people-use-feed-counters-to-show-off-top-10-ways-to-a-massive-subscriber-list.html" rel="nofollow">massive subscriber list with Feedburner or similar service.</a>
All they have to do is find a service that reports subscriber numbers for email subscriptions to Feedburner.
The link is to my response to Maki's meme on Feedbrner feed counts. Some have suggested that Feedburner no longer allow imports, at least openly in comments on my blog, but there is not harm encouraging your existing email service provider to start reporting data if they offer some kind of RSS to Email option.
Why don't email services provide a similar social proof chicklet?