My iPhone refuses to upload a picture using the flickr app. It gets through 99%, reports complete, then says "Upload failed. Try later. Try again." Not much on The Googles about this. However, I believe the answer is in Flickr’s monthly upload limit for free accounts. After upgrading to Pro, my upload worked flawlessly. To the flickr iPhone app developers, your error message needs to be altered to encourage the purchase of a pro account.
View pictures mainly from my iPhone at http://www.flickr.com/photos/mccaughan and see excellent photography from Cathy with a real camera at http://www.flickr.com/photos/domesticpsychology.