I just submitted a patch that would allow you to pass a proc to
ActionController::Base.asset_host rather than just using a %d
wildcard (e.g. assets%d.example.com). Basically this frees you from
the hard-coded assumption that you have 4 hosts numbered 0-3 and also
allows you to do smarter things depending on the asset (e.g. images
from a particular domain, stylesheets from another). The motivation
for the patch largely came from realizing that I could only set up 4
hosts (and they had to start from 0!) and also when I was trying to
answer someone's question on IRC channel #rubyonrails about
configuring the asset host (conclusion being that you had limited
options or you could monkey patch).
Please give me your ideas or any opinions for or against this at the
ticket http://dev.rubyonrails.org/ticket/10521. Thanks!