Even though the likely hood that the plastic bottle explosion and the religious folks pissed of at target about who gets to pee where, there's no proof, just conjecture.
I've made bombs from lots of stuff over the years, most of which can be found in any cabinet under any sink in any house. Like Kal even showed, a little dry ice in a bottle with a little water and... boom. Makes a heck of a loud noise, could do a little harm to the toilet seat if its in the toilet, or to a mirror if its in front of one, but the most it would really do is make someone's ears ring if they were in the room when it went off.
Until we find out what was in the bottle, I don't know if calling it a bomb (although that's what it probably and technically was) is all that accurate given the mental image that that word conjures up.
I hope they find the guy that set it off by looking at the video surveillance tapes, and make an example out of him. Even if he was just playing a prank and not wanting to hurt people.
Truth is, for it to be terrorism, all you have to do is an act that creates fear of death or injury, which it would appear this would have done given the location, and the current environment. So... charge the guy with a terrorist act. If not, then we're setting a precedence that this stuff is okay, or at least not all that bad.