In reply to nk:
Both are great cameras but it depends on what you want out of it really.
If you have a Canon 550D do you have full frame lenses for it or are you on cropped body lenses?
If you have full frame lenses for Canon and like them then i'd advocate going for the Canon.
I use a Nikon D90 and will at some point upgrade to full frame and it'll be a Nikon ( I like the metering and find them easy to use). I'd argue the Nikon JPEGS look marginally worse out of the camera than Canon ones but you can get amazing images with very little processing anyway.
I'd discount the built in flash as a feature they generally arent worth using at all- better off to buy a proper speedlite from somewhere. Strobist blog has some good recommendations for cheap high specced third party models.
One point to note if you are leaning towards the Nikon is that the D700 has been due a replacement for a while and that will probably cause a bit of a price drop.
Both are great cameras so you cant really make a wrong choice here