Robert Kirkman posts on his blog that Ultimate X-Men #93 is his last issue on the title:
A few weeks ago series editor Ralph Macchio and I were discussing what I would do after the upcoming APOCALYPSE story arc. This arc is something my run on the book has been building toward since I was given the regular writing assignment with issue 75 (issue 66-74 was supposed to be a fill-in arc). Everything I’ve done in the book thus far has been building to this and the thought of continuing after I’d completed my grand plan seemed like overstaying my welcome, frankly. I will have said everything I wanted and done everything I set out to do… so it seemed the time was right.
He adds that we should be hearing about what’s next for him “very soon.”