The record isn't 100% official yet, but Ericsson 4 broke the 600 mile barrier yesterday in the 08-09 edition of the Volvo Ocean Race. The previous best 24-hour run for a monohull was 562.96 miles, which was set by Seb Josse and the crew of ABN AMRO TWO in the 05-06 edition of the VOR. You can read all about it HERE.
This comment ought to give one pause when considering just what kind of a challenge these people face. "Winds approaching 40 knots are one thing, boisterous seas of eight metres in pitch darkness quite another."