"Joe Quesada was asked if the Ultimate Universe and the Main Marvel Universe would ever cross over and he replied no. Quesada said he'd rather close down one universe than have them cross over because it meant they were officially out of ideas." -CBR Comic-Con International Day 1 Recap, July 14th, 2005
"Having Miles Morales bump shoulders with Peter Parker in the year we're celebrating Spider-Man's 50th Anniversary was just too delicious to pass up." -Marvel Editor-in-Chief Axel Alonso, April 11th, 2012
Just sayin'. (Here are the two sources, by the way --
first quote,
second quote.)