Although multitasking is a feature that a lot of iPhone users have been asking about (including me) Apple seem to have held back on implimenting it for some reason or another. Therefore some great programmers have taken the matter upon themselves and they created various applications that can be used on a jailbroken iPhone to enable multitasking on your iPhone. This is however not reccommended (in my opinion) to any of the original iPhones, or the iPhone 3G since both of them might not be able to handle the load very well. The iPhone 3GS might be the best candidate for this fix.
To get started with multitasking on your jailbroken iPhone you need these applications standby jailbreak justifier and the essential iPhone app Backgrounder, which lets your designate any app to run in the background, and new task switcher interface called Multifl0w. These are available from the Cydia installer, however the Multiflow app is not free and would cost you something around 4Eur.