Just got some interesting info from The Stilt at MuroBBS
He's been testing a HD3870, and like pretty much all reviewers who have OCd the card, reached ~860MHz.
He said, that ATI has already released a new BIOS to fix an issue with the current/review cards - they have the PLL VIO divider set wrong, which prevents the cards from reaching over 862MHz on core. However, most of the retail cards should carry the new BIOS, luckily.
He promised to report with the new BIOS on OCs tomorrow