From USA Today,
Mysterio, the nefarious magic man who has been a Spider-Man villain since the hero's earliest days, is the guy who finds a rift between the regular Marvel Universe and the alternate-reality Ultimate Universe, a plot device that is central to the upcoming Spider-Men.
Beginning in June, the five-issue Marvel Comics miniseries by the Ultimate Comics: Spider-Man creative team of writer Brian Michael Bendis and artist Sara Pichelli will feature the first meeting of Peter, Stan Lee's original-recipe web-slinger from The Amazing Spider-Man, and Miles, Bendis' half-black, half-Hispanic teenage hero.
Bendis says fans who have been following Ultimate comics religiously will enjoy the Mysterio reveal. The villain has been making trouble in that universe for a while, traveling incognito to and from the regular universe.
The rest in the link.
I guess this is what happened to Ultimate Mysterio...