IN LATE NOVEMBER, TONY HUCK, CO-head of sales and trading at brokerage house Investment Technology Group, got word that a competitor, Credit Suisse, was allowing its clients to access his firm's popular Posit block-trading network at cut-rate commissions. Credit Suisse, using a computerized algorithm called Guerrilla, was scanning Posit for available shares and charging clients half its rival's rate of 2 cents per share for executed trades. The move was costly for Credit Suisse but gave its customers less incentive to go direct to Posit. Huck called Credit Suisse electronic trading chief Dan Mathisson and asked him to stop. Mathisson declined, and Huck promptly barred Guerrilla orders from the Posit system. Neither man will comment on the incident, though the two firms have since begun talks aimed at allowing Credit Suisse customers to once again trade with Posit orders. Squabbles like these are playing out all over the suddenly topsy-turvy...