Backgrounder is a Cydia app for multitasking in jailbroken iPhone / iPad / iPod. The app was updated to version 1.0.0-1 which now supports iOS 4.2.1. With Backgrounder you can manage apps that should continue running or the apps to quit.
Changelog for Version 1.0.0-1:
MOD: Updated for iOS 4.2.1.
MOD: Changed method used for preventing installation on unsupported iOS versions.
MOD: Changed version numbering due to move from Google Code (svn) to GitHub (git).
Backgrounder is available in Cydia via BigBoss repository for free.