Buy Destiny digitally on a PS3 or Xbox 360 and you will get the PS4 or Xbox One version included in the price before January 15th 2015. This has already been mentioned by silverrat23 in the forums but I think it deserves a mention as a deal too. Really worth considering buying digital for this one if you currently have a PS3 or 360 and intend on upgrading to a PS4 or Xbox One in the future, as this offer is not available for physical copies. Also, if you have both last and current gen consoles at home attached to the same PSN or Xbox Live account then you are essentially getting two copies of the game for the price of one, as you can continue to play the PS3/360 version after you upgrade to the PS4/XOne version. There are many better deals if you only really plan on playing Destiny on one console, but for those with a need for both this is probably your best value option. Personally, I'm looking forward to playing primarily on my PS4 while also playing online against friends with PS3s.
Full details and answers to frequently asked questions are at the link, or a few have been copied below:
Q. When I buy/redeem the Destiny Digital Base Game, Digital Guardian Edition, or Expansion Pass for PlayStation 3, how do I get the free PlayStation 4 version?
A: When you buy/redeem the Destiny Digital Base Game, Digital Guardian Edition, or Expansion Pass on or before January 15th, 2015, you can download the PlayStation 4 version from the PlayStation Store for free by doing the following:
Log into your PlayStation 4 using the same Sony Entertainment Network Account (PSN ID) used to buy/redeem the Destiny digital content on PlayStation 3.
Navigate to the Destiny content page or search for the corresponding item you purchased previously on PlayStation 3.
Add the item to your cart and complete the transaction.
Wait for the download to complete.
Note: Expansion Pass content (which includes Expansions I and II) will only be available for full download once released.
Q. When I download the free PlayStation 4 version(s), will I still be able to play it on PlayStation 3?
A: Yes. Your Sony Entertainment Network Account (PSN ID) will retain licenses for both consoles.
Q. When I buy the Destiny Limited Edition or Destiny Ghost Edition on PlayStation 3, how do I get the free PlayStation 4 version of the Expansion Pass?
A: Log into your PlayStation 3 and redeem the voucher for the Destiny Expansion Pass in the PlayStation Store on or before January 15, 2015.
Log into your PlayStation 4 any time after using the same Sony Entertainment Network Account (PSN ID) used to redeem the Destiny Expansion Pass on PlayStation 3.
Navigate to the Expansion Pass within the Destiny content page in the PlayStation Store.
Add the item to your cart and complete the transaction and wait for the download to complete.
Wait for the download to complete.
Note 1: Expansion Pass content (which includes Expansions I and II) will only be available for full download once released.
Note 2: A physical or digital PlayStation 4 version of the Destiny game will be required to play this content on PlayStation 4, sold separately
Q. Will your Guardian's, progress, stats, and gears carry over when you download and play the PS4 version?:
A: Yes. Whether you participate in this program or not, your Guardians, progress, stats, and gear are shared within a console family (PlayStation 3<->PlayStation 4) and will be waiting for you when you transition to next-gen.
Not buying it but that's great to see.