Well, it's about time. Touted back in March as one of the pillar apps to benefit from the new multitasking API, Skype has finally been updated to iOS 4 compatibility. We're seeing crisper, more retina display-friendly graphics and, more importantly, background VoIP (not for iPhone 3G, naturally). A notification window will pop up when you're called or messaged, and you can use other apps while on the line chatting to international friends at reduced rates. Even on the lock screen, if you receive a call, sliding to unlock will take you immediately into the call. Video chat still isn't here -- a darn shame given the lack of other options now -- but now you've got even more of a reason to rack up those rollover minutes on AT&T's voice plan. Download away!
Update: Remember when Skype was talking about charging extra for calls over 3G starting this month? Straight from the horse's mouth: "we no longer have plans to charge a supplement to make calls over 3G."