Patch 5.1: Brawler's Guild invites not available for 24 hours
The Brawler's Guild is included in patch 5.1, but the invitations will probably not be available on the black market auction house for 24 hours. It is unclear if the NPCs who drop them will start dropping right away or if the drops will be delayed as well. Nethaera has the details.
To paraphrase @BlizzardCS, don't make a mad dash to the BMAH today. However, they do say "may", which could mean that invites will be available before tomorrow. No need to camp, though. The number of invitations is only limited to those who can afford them.
Will you be fighting in the Brawler's Guild? Or don't you want to talk about it?