If you are having an iPhone 3GS, is no rush to get an iPhone 4 yet. The reasons are just simple, I bet you still have contract with the telco which came with 3GS.
The iPhone 4 A4 cpu is using the ARM cortex A8 core cpu, which is same with the iPhone 3GS. Sure iPhone 4 may improve performance in graphic display and power management.
The iPhone 4 is only available officially in 2 Asia countries (Hong Kong and Singapore), and coming soon and available in other Asian countries, but who know when? Some telco like Singtel already sold out its iPhone 4.
No rush to get an iPhone 4 if you are using 3GS now. If you need battery life improvement, get iPhone 4. If you are iPhone die hard fan, then you must get an iPhone 4.