The problem is caused by how front brakes are mounted on a suspension fork-on my bike in particular, Deore XT brakes on a Manitou fork. Unlike sidepull road brakes, the pads on the Deore XT extend back past the fork, so they do not open up enough to clear a big tire because they hit the inside of the fork.

My point to the OP was to be aware of this before deciding to go to a bigger tire. I haven't looked at every brake design on the market, but this is probably a common problem with big tires on mountain bikes. I don't remember this problem on the non-suspension fork on my original 1980's vintage Stumpjumper, probably because the fork tubes weren't as large in diameter as the newer suspension forks.