You must be running a ridiculous amount of things in the background.That's just because they cannot run more then 1 app at once. My jailbroken iPhone using a radio streaming application uses ALL the memory, when I put it in background and start Safari, memory runs out and it closes the radio streaming application. I can listen and check my mail, or use ebuddy, but loading safari would require more memory then the iPhone have.
I can do what you can't without issue on a 3G of all phones.