Debian Sarge server frozen (em64t-p4-smp)

[Simon <greminn@gmail.com>]

Hi There, We have a mail server (postfix>amavis>dbmail) running debian
sarge (2.6.8-9-em64t-p4-smp) running dual xeons with a tyan
motherboard and 3ware hardware raid5. Everything has been running fine
up to now, but in the last week it has frozen twice. Completly stuck
needing a hard reboot to restart (ctl-alt-del dosnt work). Here is a
screenshot of the stuck screen (no ping, nothing at this point):

http://gremin.orcon.net.nz/console.jpg

Would someone be able to take a look and give me a clue here?

Simon