Another portion of the CSS code, this time talking about pre-orders, only mentions Xbox One, PS4, and PC. This could suggest that Fallout 4 will be exclusive to current-gen, or that only the current-gen versions will be available for pre-order at the time of tomorrow's announcement.
We are currently seeing something similar with Call of Duty: Black Ops 3, as Activision has only announced and taking pre-orders for the Xbox One, PS4, and PC versions, but maintains that "actual platform availability and launch date(s) to be determined."
With the whole transition to Xbox One and PS4, it's not uncommon to see the main developer focus on current-gen (Xbox One/PS4), while outsourcing development for the last-gen versions (Xbox 360, PS3) to another studio.
It's possible tomorrow's announcement will only include the Xbox One, PS4, and PC versions, with a later announcement confirming last-gen versions.
Earlier today, Bethesda essentially confirmed Fallout 4, teasing the impending official announcement which will be made tomorrow.
While we'll have to wait for tomorrow for official details, newly discovered information reveals that Fallout 4 will not be exclusive to current-gen systems. The eagle-eyed detectives over at NeoGAF have scoured the teaser site's source code, discovering in the CSS lines of code that suggest Fallout 4 will be available on last-gen systems as well.
According to the CSS file, Fallout 4 will be released on Xbox One, PS4, PC, as well as Xbox 360 and PS3. And because I'm sure one person cares, there was no mention of Wii U.
You can see a snippet of the code below, with the part that mentions Xbox 360 highlighted.
As the Gaffers also point out, the list of games Bethesda will be bringing to E3 also matches up accordingly. Battlecry (free-to-play) and Doom (action) are both only listed for Xbox One, PS4, and PC, while the Role-Playing game, presumably Fallout 4, is listed as cross-gen for Xbox One, PS4, Xbox 360, PS3, and PC.
Make of it what you will. If we don't find out tomorrow, we'll almost certainly know for sure come Bethesda's E3 press conference on June 14th.