Apple’s new 2011 MacBook Pros boot into 64-bit mode by default. Previously only late model Mac Pros and Xserves booted into 64-bit mode, everything else booted into 32-bit mode.
Cisco VPN client versions 4.9.01.0280 and older will not work on a Mac booted into 64-bit mode, giving an error 51 at launch. Until Cisco releases a new version that is 64-bit compatible, Macs that boot into 64-bit mode will need to be manually booted into 32-bit mode in order to run Cisco VPN. See this page for instructions.