One of the new features of iPhone 4 is "FaceTime" video chatting. If you're on a call with another iPhone 4 user and both of you connected to WiFi, you'll see a FaceTime button in the bottom row of call options, that replaces the "Hold" button found on other iPhones.
So how do you put a call on hold now? A user emailed Steve Jobs, the Jobs replied:
"Hold doesn't do anything more than Mute".