SJ Games came up with a perfectly good world in their original edition of GURPS Time Travel Adventures.
It was expanded slightly into its own book, Technomancer.
In short the Technomancer/Merlin world is our world except where Oppenheimer quoting "I am become Death..." casts a spell when combined with the Trinity Event to open up the manastorm.
This also floods The Earth with magic.
Like I said, a perfectly good world.
The original setting was set in the year it was published, 1998.
There was an additional setting for the world, set during the Vietnam War titled Funny New Guys.
I did some expansion too, projecting a win for Rhodesia.
They've never updated either time frame for the 4e rules.
I just got done converting the racial templates from Technomancer and the character templates from Funny New Guys. If you want them, gimme an email in the comments section.