If you have an iPhone 3G, or a second gen iPod Touch, I’m really sorry for you guys. You won’t be able to use the new multitasking feature of iPhone OS 4.0 with them. Actually Steve Jobs mentioned that these devices will be able to do “many things”, but multitasking won’t be one of them. Clearly we know that they can indeed run backgrounding apps with jailbreak extensions, but Apple doesn’t seem to care about it. Also, no mentioned about first gen iPhones and iPods.
Anyway, I bet the jailbreak community will come up with new solutions and enable multitasking (which is there in the firmware) even for these devices. Until that time, you might want to upgrade to a 3GS. Or perhaps the new models that will come out this summer.