I feel almost embarrassed that as a huge, huge Rock Band fan it took me three whole hours to realise that Rock Band 3 now has a concrete - and fairly imminent - release date. That date, ladies and non-ladies, is October 26th.
While the specific press release announcing this solely mentions North America, Harmonix had previously confirmed that the game is set to have a simultaneous worldwide release. What this means though, is that while you lucky Americans/Canadians get the game on that Tuesday 26th, we in Europe will still have to wait a gruelling, tortuous three days before it releases on our standard release day of Friday (and Australians on the Thursday, I believe!).
EDIT: CVG have confirmed that the UK release date is indeed October 29th. I would assume that's the same for the rest of Europe too.
Also for you lucky North Americans, Harmonix have announced three pre-order bonuses for the game, outlined below.
GameStop Pre-order Opportunity:
GameStop shoppers who reserve Rock Band 3 for Xbox 360 in advance receive three downloadable Rock Band songs at launch:
• "Burning Down the House" by Talking Heads
• "My Own Summer" by Deftones
• "Blue Monday" by New Order
Amazon Pre-order Opportunity:
Amazon shoppers who reserve Rock Band 3 for Xbox 360 in advance will receive access to an exclusive in-game guitar. Additionally, for a limited time only, PlayStation 3 and Xbox 360 owners who pre-order Rock Band 3 in advance on Amazon will receive $10 off a future videogame purchase.
Wal-Mart Pre-order Opportunity:
Walmart.com shoppers who reserve Rock Band 3 for Xbox 360 and Wii will receive a $10 eGift card for a limited time.
Here's a picture of that exclusive Amazon-only in-game guitar, courtesy of RockBandAide.com:
And don't worry, you can pretty much guarantee that those three DLC songs will hit the standard Marketplace at some point too, presumably after the game's launch.
We've got the full list of Rock Band 2 achievements - check the list for guides to unlocking them.