ive gone from an xt to a 30d to a 40d in the past six months. i really, really am impressed with the 40d. wicked image quality even at high ISO; the 'live view' function is a godsend for any and all tripod work, whether landscape or macro; i like the various bracketing options; feels nice in the hand; pretty clear menus and so forth. the sRAW and RAW + JPEG output options are neat. right now, i cant see any way to step up in the canon line for under $4000. (the 40d gives as good an image, if not better, than the full frame 5d).
that said, ive been hearing some incredible stuff about nikon's new full frame (the d3?) and its crop frame cousin, the d300. supposedly the d300 has almost all the improvements of the d3, but at a fraction of the price. people are scratching their heads saying, "why did they even bother to release the d3 if the d300 has all the important bells and whistles but is a fraction of the price?"
but its all a game of minor, relatively incremental improvements. today nikon is a quarter step ahead of canon. tomorrow canon will be back in the lead, by a nose. repeat ad infinitium. you cant go wrong with either brand. i understand nikon glass is more expensive, however. look at the pentax k20, i hear its pretty nice too. i woudlnt bother with any other DLSR lines though...