So far it has been nice and smooth on my 3GS.
Its sitting there with 168MB of ram open waiting to multitask.
Backrounder is awesome on the 3GS.
Im running Navigon GPS, along with Pandora and I even drop a few text messages all at the same time without even breaking a sweat.
With the added ram and faster CPU Apple should have enabled apps to run in the backround on the 3GS models.
i still agree with Apple on this front. most iPhone users still have the older phones. even if i had a newer one - i probably wouldn't use backgrounder. it was glitchy for me and IMO wasn't worth the battery wasted.
sure i could do 2 things at once on my 2G - but it slowed everything down to the point it would just be faster to quit an reload.