They say its because of hardware limitations but I smell a rat. I have a jailbroken 3G with 'MusicControls' and it allows me to background pandora just fine.
This is just a way of forcing people to upgrade to the latest non-jailbreakable hardware.
They care about the end user experience. They don't care at all about what you've managed to get working on a jailbroken phone. My take is that they decided the experience wouldn't be good on the 3G. (They were emphatic about this point in Q&A).
Either way, it's irrelevant. I didn't buy my 3G in the anticipation of having multitasking. I bought it because I wanted what it was then. I'm upgrade-eligible soon enough, and happy to have a reason to buy a 4th-gen iPhone.
As has been noted many times, the plural of "anecdote" is not "data". Or more succinctly: "I was able to make it work on my jailbroken phone" does not imply "it would work well enough consistently enough to make the broader market happy".
The have done this before, when MMS came out they did not include it for 2G phones when they are perfectly capable of sending MMS.
Someone else pointed out that in the case of multitasking they cannot guarantee that a 3G can handle the general case, that may be true, we shall have to wait and see.
The 3GS has twice as much memory. It's taken Apple years to iron this out, and they have historically made features available on older hardware, so my bet is that 128Meg isn't enough to support common usage of this feature. It's possible that OS4 uses more memory itself.
