I start to feel that playing around the iOS 4is like treasure hunt.
Apart from all the fantastic features highlighted by Steve Jobs in his WWDC 2010 Keynote Address (QuickTime movie) and April 2010 Special Event (QuickTime movie), I just find that they have added a contact linking function in the address book.
The new feature allows you to group different entries from your address book together. That means you can group multiple contacts from the same company, or members of a family together to create a bundled entry.
Same logic as folders of apps, this one is folders for contacts.