Flickit Pro, the Green Volcano Software client for Flickr.com, was updated to version 2.1 last night, which brings full support for iOS 4 and iPhone 4. Besides improved video quality upload and overall upload speeds, version 2.1 brings the much-request background uploading to Flickr and UI optimizations for the Retina Display.
I’ve been beta testing Flickit Pro 2.1, and it works like this: in the Settings you can choose to hide / show a notification for when the upload is complete. Choose some photos, send them to Flickr, close the app and they will be uploaded anyway. It works really well.
Personally, I think Flickit Pro is the best Flickr client for the iPhone. The new and improved version makes it even better. Available at $4.99 in the App Store.