ashikase, the dev of Backgrounder and MultiIconMover has released a beta version for both tweaks with support for iOS 4.2.1. This is not an official release, and you won’t find them in Cydia. Note that both betas are untested, and meant for bug reports. But if you already updated to 4.2.1 and use the existing jailbreaks ( most probably tethered ), there’s not much you can loose.
Both updates are distributed as .deb files and, ( for most of you ) probably the easiest way to install .deb files is to auto-install them.