You can enable it with jailbreaking. Lags occasionally, but I got that with iOS4 before I JB'd so it might not be the multitasking.
BTW, I've compared using Backgrounder with 3.2.2 to using the built-in multitasking and fast app restarting capabilities of the (up to now) two developer betas of 4.2 on my iPad. The latter is an order of magnitude more stable and usable - the system very rarely kills e.g. ooTunes playing a radio station (and recording at the same time), unlike with Backgrounder. That is, I think even with the 4.0+ + JB + Multitask hack, iPT2's, iPhone3G's etc. are considerably better than using Backgrounder instead.