I believe they use Cloudflare and Prolexic to handle DDOS. I'm not sure how that applies to their trading engine though... it seems like they get flooded with huge numbers of .01 ask/bid adds and removes, and then the lag starts.
IP address of mtgox.com is 72.52.5.67.
Info from
http://whois.domaintools.com/72.52.5.67 AS32787 PROLEXIC-TECHNOLOGIES-DDOS-MITIGATION-NETWORK - Prolexic Technologies, Inc. (registered Jul 23, 2004)