I'll assume the thought of calling up Hamilton as a late season weapon for the bench has crossed Walt's mind. If he's at all entertaining the idea, I would think he'd promote him to AA until August 31st prior to calling him up. I realize he likely wouldn't bat much in september/playoffs, but I would think you'd at least want to give him some experience against the more polished pitchers in AA.
As far as if I would do it, absolutely. Like Doug said, I'd make sure he was with the team on Aug 31st so that he'd be eligible for the playoffs (would be a good time for a 15 day DL stint for Rolen to rest his shoulder if the Reds had a decent sized lead in the division). Imagine what kind of havoc his speed could play in a key situation in the 7th, 8th or 9th inning of a tight game. If he doesn't burn an option this season, that gives him '12, '13 & '14 to continue to develop in the minors which would make him 24 (?) when he "had" to be on the ML roster full time. If he's not ready to be a regular by then, even by way of the bench, then odds are that he'll never be much more than filler on a ML roster going forward. That would still give him 2 full seasons in AA & AAA, plus 1 season repeating one or the other if needed.
Sure it would be nice to have him under control a little longer when he'd likely give you something more than just speed off the bench for a late season call up or injury replacement down the road, but if you can call up a player now that might make the difference b/w winning a postseason or playoff race game or 2 this year, vs having the option to send him back to the minors when he's 24 or 25, I'll take the chance that he can help push this team a little further this season and worry about the '14 & '15 season when we get to it.
If they promote him to AA, the only downside is the risk that you take some polish off his prospect status in a trade should he struggle. His speed will still play as a pinch runner in the majors. They could always promote him to AA and decide down the road that a late season call up isn't worth burning the option.