I always have had to stop and think about this one, but for the last couple of years I've taken more note of printed word usage -- in books and with closed-captioning it seems to be invariably rendered as "hear, hear".

hear now some of the various opinions:
http://www.straightdope.com/mailbag/mhear.html
http://www.montypython.net/scripts/bruceskit.php
{Everybruce: Hear, hear! Well spoken, Bruce! }*
http://www.hear-hear.com/writing/992132745/index_html
http://lawlibrary.ucdavis.edu/LAWLIB/jun94/0186.html

*it is fascinating how many cross-threads manifest in this one link.