The new boss, Coren Direbrew, is pretty cool and I'm happy to see older content being made relevant again (especially when it's an instance as cool as BRD), but as far as I can tell from the forums and various data sites, the drop rate on the mounts is...less than spectacular. I'm seeing estimates that range from as little as 1% to as much as 15%. Blue poster Kisirani pops up to say it's "much, much easier" to get a Brewfest mount drop than it was to get the Sinister Squashling off the Headless Horseman. Wowhead lists the Squashling as a 2% drop, so maybe the 15% figure for the Brewfest mounts is more accurate.
The mount might be a crapshoot, but you can cover the other bases on an upcoming Achievement and title by planning ahead:
While the Brewmaster Achievement and title won't become available until Brewfest 2009, one of the needed sub-Achievements is Brew of the Year, which requires you to obtain and drink each of the 12 Brew of the Month Club beers. You become a member of the club by forking over 200 Brewfest Prize Tokens, and you'll need to get it now in order to have all those brewskies under your belt in time for Brewfest 2009. You'll also need to have 3 pieces of Brewfest clothing squirrelled away so you can get drunk and make a pest of yourself /dancing in Dalaran for Disturbing the Peace once Wrath hits.
So, if you're doing Brewfest this year with an eye toward the Brewmaster title and Achievement, make sure you've gotten the following:
Brew of the Month Club: 200 tokens
Brewfest Dress/Regalia: 200 tokens
Brewfest Boots/Slippers: 100 tokens
Brewfest Hat: 50 tokens
Wolpertinger pet: 50s (if you don't have one from last year as a quest reward)
The total comes to 550 tokens plus 50 silver. All of the other sub-Achievements are things that have to be done while Brewfest is on (you can't buy stuff like Succulent Sausage in advance as its 2-day duration guarantees it won't be around by the time Wrath comes) and are also considerably easier, so you don't have to worry about it until Brewfest 2009.
The catch? You still need a Brewfest mount (Have Keg, Will Travel) to complete the full Achievement, so if you don't have a Ram from last year, keep running the BRD event and pray the mount drops (although it's certainly possible that Blizzard will change how you get the mount again next year, so don't go nuts if you don't get a drop). The Brewmaster Achievement itself is part of the even larger Achievement What A Long, Strange Trip It's Been, so good luck, and start saving those tokens!
**Edit: I probably didn't make this clear, but I don't advocate saving tokens from this year for next year; as commenters have observed, the tokens this year expire after 21 days so there's no point to doing that. What I do suggest is that, if you're keen on getting as much of the Brewmaster Achievement done as early as you can, these are the tokens and items you'll need to get at this year's Brewfest. Either way you'll need to get the Brew of the Month Club Achievement started now to be finished in time for Brewfest 2009.**