Apple has just released their much awaited iOS 4.0.1 update that is targeted to fix the iPhone 4 reception problems. However the new update will only change the formula for calculating the signal strength and its impact on signal quality would be minimal. The new update is available for iPhone 4/3GS/3G models.
Luckily, the new firmware does not update the iPhone baseband which lets you run on previous unlock in case you’re not using AT&T network. Jailbreaking support for all these devices are expected soon.
You can download the supported IPSW file for your iPhone from the links below.