config.asset_host being ignored

I've set my asset_host (Rails 3.1.3 app) to a subdomain that points to
a cloudfront cdn. However, when I load the page all my asset helpers
are still producing relative URLs to assets instead of using the fully
qualified asset_host.

My SCSS files are using the CDN via the image_path, so it's only the
helpers in my views that aren't working.

Any ideas?