How to cache content per user? Is there a way to cache content with the (% cache %} tag on a per-user base?

If user A is logged in, his content gets cached. User B logs in, but his content should not interfere with that of User A.
In the upcoming Craft 2.2 release, we've added a using key parameter to the {% cache %} tag for for situations like this.

So you'd be able to do something like this for per user caching:

{% cache using key "myImportantUserCache" ~ currentUser.id %}