I'm jailbroken with 4.2.1, yes (which is a little snappier than 4.1). Everything works fine and I'm generally resigned to the slowness of 'old' tech with little ram in this context. It's just that after finding an app that immensely speeded up opening my folders by switching off the animation, I'd really like to find a similar way to switch off the home screen animation too, which is completely superfluous to my needs. So far no joy, but I'm asking around with jb devs to see if someone wants to crack this one too.
Multitasking has become patchier to use in 4.2.1 for jailbroken 3G's, at least - but then there's only really room for 3-4 apps running at once anyhow