It's not a 45 degree incline, by any means, but we got our mecanum to go up a ramp a couple years back. The first try didn't work, so we fixed up a few things (hence the clip of a student welding the frame) and then tried again.
The catch is keeping all four wheels in contact at the same time... but here's proof that a "standard" mecanum can climb at least a shallow incline.
http://www.youtube.com/dtengineering...12/gyn9e0cT3-A
Jason
P.S. If you want omni directional drive ability, plus climbing... perhaps some form of crab drive is what you're looking for?