It is likely that Apple will release iOS4 for the iPhone today, perhaps around 10 PST. iTunes will update you automatically on the date listed in Summary. To proactively update your iPhone, in iTunes click on your iPhone (under Devices on left column). Then in Summary screen click "Check for Update".
The feature I am most looking for is folders. That will allow me to place icons for all my iPhone Apps on the iPhone . The other feature I look forward to, multi-tasking, will evolve in its usefulness overtime, as developers conform to iOS4 API.