At your cache layer (e.g. Varnish), you would need some VCL to collapse all the hundreds of User-Agents options into something sane like "X-Device", feed this header to your backend, and vary the content based on that instead. This way you can cache just a few representations per resource (e.g., "Desktop", "Mobile", "Tablet", etc.) instead of several hundred. The problem, of course, is maintaing this list of User-Agents.
On his mobile example, it's much easier to simply be pragmatic and serve content for different devices at differents URIs/domains altogether. It doesn't break if you can follow some kind of convention, such that the URLs from one map directly to the other (for instance, mysite.com/news/article -> m.mysite.com/news/article) and then you issue redirects accordingly.
On his mobile example, it's much easier to simply be pragmatic and serve content for different devices at differents URIs/domains altogether. It doesn't break if you can follow some kind of convention, such that the URLs from one map directly to the other (for instance, mysite.com/news/article -> m.mysite.com/news/article) and then you issue redirects accordingly.