Make sure debug=false is set in the <compilation> section in config.
Then the web resource URLs will be cached (like the client libraries).
The “t=” in the web resource URLs represents the assembly's last modified date.
As long as you don't change the atlas assembly, the same “t=” should be generated. Proxies should get cached as well.. They are retrieved from the server as the browser parses the page, and loads referenced scripts.
This is completely orthogonal to server lifecycle events such as Page_Init. There are some enhancements for caching in MS AJAX RTM that make the scenario even better.
Thanks to Nikhil Kothari for my edification.