BusinessWeek Online - This is not an easy time to nurture a new generation of corporate leaders. Training budgets are being slashed while a depressed housing market has made it harder to move around for better job opportunities. And yet the need for top talent is growing. A record 1,484 U.S.-based chief executives left their jobs in 2008, according to outplacement firm Challenger, Gray & Christmas. Many more could step down this year as losses mount and executive angst runs high. …