The current suggestions have all been great, and like most I think that option 2 may be confusing for the user in terms of continuity with other systems such as iOS where the icon shown represents a clickable element to clear the field.
One suggestion I haven't seen above is temporary background changes to the parent of the input. This serves 3 purposes:
1) The area with colour change is larger. It draws the user's eye in (assuming it's visible)
2) The area changing colour is going from white to a colour rather than one colour to another. I work on a lot of gov systems and we have to consider colour blindness in everything we do.
3) It doesn't take away from your styling once the colour has animated away.
I would however like to point out that this method must be backed up with something like option 1 or the eBay/twitter methods - once the animation is gone the user need to stay aware of which fields require attention.
Something that just came to mind - what about option 2 with a different icon? Then you don't have the consistency issues. Something like a yellow warning triangle with exclamation mark in it would work IMO.