Breaker voltage ratings are MAXIMUM, anything less is technically OK. Lower voltage ratings are usually offered because they are slightly less expensive and/or smaller (spacings between terminals can be lower, smaller arc chutes etc.).
But as kingpb said, there may be an issue with mounting it in a panelboard (if that's what you are doing). In other words a 120/208V rated panel may not have a UL listed method of bolting in a 600V rated branch breaker. The only way to tell is to look at all of the breaker types officially listed for that panelboard.
If it's stand-alone, like in a control panel, then no problem at all, but I would add a very clear label on the panel surface near the terminals that the feed is 208V so that someone in the future doesn't think it's 480V by looking at the breaker rating. Just a courtesy for the next guy though.