Panasonic has a model with an iPhone dock that you can use to transfer pictures directly. Actually any frame with a USB socket should work; just plug the iPhone in with a sync cable and it will show up as a USB drive with a DCIM directory containing pictures, just like a camera memory card.
Some frames let you add pictures by emailing them to a special address, e.g. Kodak wifi frames have this feature. You could email pics from your iPhone, this would be handy particularly when you are not near the frame (e.g. email pictures to a family member).
A WiFi frame with the ability to pull from your iCloud Photo Stream would be neat, my Apple TV does it & displays Photo Stream pictures as a screen saver. I haven't seen any digital frame products with that capability yet.