The permissions are cached just fine and properly flushed when updated in most cases (if you use the API rather than hitting the db directly that is). The problem is that if you add or remove someone from a group (or add or remove a permission on an object for a group) the cached value can then be invalid.
This is the case for pretty much any inherited permission.
Properly maintaining the cache in that circumstance is
very expensive and consequently no attempt is made to do
it.
So the short answer is that if you have permissions set up
in a particular way on your site it *might* work but
but generally there are a lot of cases where the cache
is out of sync with the DB.
With Don's permissions checking speedup work it is
hopefully not necessary for most sites anyway.
