Rails 3 Conditional Inclusion of Javascript files

Question

I\'m trying to figure out the best way to include the following in my head in rails 3:

Conditional comments, rather than serverside useragent scraping, are the best way to go about serving additional CSS/JS assets to IE/specific versions of IE. Some proxy servers modify the User-agent header which means that some IE users might not have the excanvas file inserted into the :defaults expansion. Inserting it like this means all IE users will be served the file, not just those where the user-agent matches a typical IE useragent.