This post has a link about cell phone forensics:
http://boards.courttv.com/showthread.php?s=6481f0e123e384af37a9208ccbc37009&postid=8054244#post8054244
I would imagine, after reading this, that anyone attempting to change the timestamp on a cell phone image would be confronted with multiple problems. Not only does the image file have imbedded data, but the phone itself probably has a record.
Anyone trying, on short notice, to tamper with this stuff would be risking getting caught, and getting caught would be like signing a confession.