What user to defaults execute as?

Do default expressions execute with access of the user doing the insert
or the owner of the table?
What I was thinking was that the owner of a table may want to allow people
to do inserts into a table and update a sequence through a default expression,
but not let people change the sequence directly.
A quick look under create table (in the 7.3b3 docs) didn't turn up any
information and I don't remember running accross anything covering this
previously.