Of course it can always change at any time, but so far as I know the BB dates have always been the same at all resorts.
The flyers might look different at each resort, and might advertise different resorts you can BB to, but when you call the extension 8844 from your room you can ask about ALL the BB's being offered across the categories during your stay so you can pick the one you want to book, like this compliation of the current BB's being offered at different resorts:
Get 20% off a Select Disney VALUE Resort: 1/3-1/14/2010, 1/18-2/11, 2/15-3/27, 4/11-7/1, 7/5-11/23, 11/28-12/24/2010
Get 35% off a Select Disney MODERATE Resort: 1/3-1/14, 1/18-2/11, 2/15-3/27, 4/11-7/1, 7/5-11/23, 11/28-12/24/2010
Get 40% off WILDERNESS CABINS: 1/3-1/14, 1/18-2/11, 2/15-3/27, 4/11-7/1, 7/5-11/23, 11/28-12/24/2010
Get 40% off a Select Disney DELUXE Resort: 1/3-1/14, 1/18-2/11, 2/15-3/27, 4/11-7/1, 7/5-11/23, 11/28-12/24/2010
Get 45% off a Select Disney DELUXE VILLA Resort: 1/3-1/14, 1/18-2/11, 2/15-3/27, 4/11-7/1, 7/5-9/30/2010 (Offer excludes 3 bedroom grand villas) * Note: another nice DISer added these dates for the Villas 8/15-11/23 and 11/28-12/16/2010)
In this example, the resort guests staying at POP might get a flyer showing the 20% off Values, but they can book the 40% at a Deluxe if they ask. JMO (and there is probably some really good $$$ reason Disney Marketing does it that way

), but I think they should better explain about your choices instead of having a tiny sentence "
Ask About Other Resorts, Lengths of Stay, and Availability" on the BB flyer. I think most people think what's advertised on their BB flyer is all they can book.