for me its financial more than anything. if my want to change brands was strong enough i would, but i cant see the point in changing for someone like me, unless the cost of change over was minimal.
to give you an idea on the gear i have and its dollar value on RRP
D700 - $3999
16-35mm - $1999
50mm - $229
80-200mm - $1799
70-300mm - $699
300mm - $7999
speedlight x2 $499
6 batteries - $100
thats around $18,000
if i sold it all today i would maybe get $10,000. to buy canon gear that would tempt me to change would probably cost $15-20k
for someone like me who's not made of money, its an expensive exercise.
nikon/canon is as good as holden/ford. they both do the same thing in their own way.
i prefer canon image quality in their colours and contrast, but nikon have better natural colours and better low light performance.
nikon menu's and controls are much nicer to use and more ergonomic.
if i was starting from scratch 6 months ago i would have jumped on canon, if i was starting today, id jump on nikon. for what i do, the D4 and D800 are amazing looking equipment.
that said, when canon revamp their entire range this year, it may change again. point being, they will always leapfrog each other on technology and performance, but realistically, my 3yr old d700 is more than i really need. (wont stop me buying the d800 though