I am doing some work with JRuby that may need asynchronous HTTP support.  
There doesn't seem to be much documentation about this online, so I did 
some digging to see how the existing Rack async servers do things, and 
wrote up some documentation for myself.  I thought maybe others would find 
it useful: 
http://polycrystal.org/2012/04/15/asynchronous_responses_in_rack.html

I also found some posts from a couple years ago implying that 
'async.callback' will never be a part of the Rack specification: 

https://groups.google.com/forum/?fromgroups#!searchin/rack-devel/async/rack-devel/fu58paleLJg/frwK57QHNn4J

Is this still the case?  I must say I'm a bit disappointed by this 
discovery, but I don't fully understand all the issues (in particular, my 
documentation above ignores interaction with middleware).