Well, Scott (Fellow Nikon D90 enthusiast)brought up a valuable point today, together with reason and choice for a second DSLR body. First off, a second body minimize the need for one to swap lens while shooting, giving one higher up-time to catch that right moment when it surfaces.
With that, the price you pay is obviously a heavier encumbrance. And in return, one would hope to perhaps obtain a more balanced view about photography - even a better appreciation of the strength of other camera makers (if any). I am definitely not in favor economically and realistically to own two sets of identical reach lenses if I were to move with two camera makers concurrently.
Now, let's just take Canon as a potential candidate for our discussion...Objectively speaking, Canon seems to offer a higher ISO sensor firmware, even though many believe its nearly impossible to use them at anything beyond 3200. Secondly, her EF range of 70-300 F/4 USM starts at an affordable $900! This favorable price variance seems to also extend to her EF 50mm F/1.4 USM, EF 85mm F/1.8 USM. Perhaps, its an apple and pear comparison but they are after all, still fruits!
Shutter frame rate is another consideration that seems to warrant some attention. The Canon xxD series seems to still maintain a over 6 FPS ratings while upping the ante in the shutter speed department with a 1/8000 second snap rate. Magnesium alloy body is an optional feature to me as opposed to wanting a decent weather sealing property.