Would a cached page be considered, solely by definition, RESTful?
If a page is cached or not is in my opinion completely irrelevant. Page caching is transparent to the user and the content, he doesn't know if it is and once the cache expires, he might get something different on the same page. A page could for example be a view that randomly displays a node. Enabling page caching (and the node only switches e.g. every 5 minutes) doesn't change anything.

RESTful is about having a 1-1 mapping between an URL and a resource (or maybe better: a representation of that resource, e.g. a different format). IMHO, it doesn't matter if..


authorization is necessary to have access to that resource (anon vs. authorized users)
Only a part of that resource is visible (e.g. someone with additional permissions might see more information)
If the displayed information changes, as long as it's the same resource on the same URL (e.g., a node could have an access counter that increases every time you view it.)
If there is some sort of caching involved, either page or partial caching.