Things has great-looking apps for iPhone and iPad, and it supports a good desktop sync (WiFi only). However, no cloud sync is available as of 12 April 2011. Look for it after July 2011. See their cloud sync article and follow-up posts. The new cloud sync will require a full rewrite of the Things WiFi desktop sync, so read carefully before jumping on board.
Things for iPhone/iPad is only suitable for use with Things on the Mac.