Just tried. Proswitcher was no good. Crashing springboard right away. Backgrounder doesnt seem to Be any problem though, but ios4s taskmanager doest play together, so either need an update of backgrounder that make the ios4 taskmanager play together or a new taskmanager that is ios4 comp.
Frankly I don't *know* that Backgrounder/Proswitcher have any advantages over the native multitasking on iOS4. I've been hesitant to upgrade my 3Gs so I've not tried it myself.