MVCC & indexes?

Ok, so MVCC is the best thing since a guy put a round stone on a stick
and called it "the wheel", but I've seen several references on this list
about "indexes not being under MVCC" - at least that's how I read it,
the original posts were explaining why indexes can't be used for solving
MIN()/MAX()/COUNT() aggregates. Is this correct?

In particular, I'm trying to find out is there (b)locking involved when
concurrently updating and/or inserting records in an indexed table. My
guess is that, since PG does copy+delete on updating, even updating a
non-indexed field will require fixups in the index tree (to point to the
new record) and thus (b)locking.