Note that, in computer systems, the top conferences are more important
than even the top journals.  The top conferences use a rigorous review
process in which (3-7) program committee members evaluate all
submitted papers.  Furthermore, these conferences often perform
"shepherding" of papers, in which program committee members supervise
the revision of accepted papers according to the reviewers' comments.
In terms of acceptance rates, the top conferences in computer systems
typically accept 10%-20% of the submitted papers.  For more details
see <a href="http://books.nap.edu/html/acesc/"> this 1994 National
Academy of Sciences report (Chapter 4 in particular)</a> and <a
href="http://www.cra.org/reports/tenure_review.html"> this Computing
Research Association Best Practices Memo</a>.