The official Flickr iPhone application was updated last night to include support for iOS 4 multitasking (fast app switching) and uploads in the background. It’s available for free here.
The 1.2 update also includes support for HD video uploads, Twitter integration with flic.kr short URLs and better full-screen browsing features. Sadly, there’s still no support for Retina Display.
What are you waiting for, Flickr?