The main MPEx frontend, http://mpex.com is undergoing a DDoS attack, saturating the traffic uplink and causing  the site to be unavailable. The MPEx is unaffected and is still processing trades. You can get to it through a number of web proxies, including this one: http://mpex.coinbr.com.

Mircea Popescu just redirected traffic to whitehouse.gov as a gag (and it might actually give the attackers pause), since it'll be a while before he can solve the DDoS problem.