MacRumors is reporting the iPhone 4 is packing a (relatively) whopping 512 megabytes of RAM. To put this into perspective, this is four times the RAM of the original iPhone 3G — and twice as much as both the 3GS and the iPad.
The RAM upgrade was actually announced by Apple at the recent WWDC. (If you're a registered Apple developer, look up "Session 147, Advanced Performance Optimization on iPhone OS, pt 2.") This is somewhat surprising, as the House of Jobs usually avoids publishing ANY kind of technical specs for their iDevices.
The memory boost might explain why previous iPhones didn't support multitasking — and why the new iMovie app will ONLY run on the i4 and not the iPad.