Is a bounty legal? Can lynch mobs go unhindered in today's society?
I think the offer of a bounty is legal, protected by free speech. But, if a person tries to detain Zimmerman, they are at least committing a tort for which they face civil liability, and might be committing a crime.
One way to test it is to offer bounties on certain liberals - say, Maher, just to pick one at random.
As for the ability of a lunch mob to go unimpeded, I think that depends on several factors. Black and liberals have pretty much free latitude. The authorities won't act in the case of liberals, because they are in the same camp; and won't act against blacks for fear of giving an excuse for violent reaction.