I was going to suggest the HP's myself, but I've had bad experiences with their quality lately, more plastic and less quality control.
When you say "desktop replacement", are you really looking for a true desktop replacement grade machine, or just a notebook with a 17" display?
You'll have a hard time finding a true desktop replacement at 5lbs, the Dell M2010 for example is a whopping 18lbs...
A Dell M1710 may be an option for you and fits the budget too.
One more tip: don't always fall for the built in webcam option, the one in the HP's is just plain nasty and while it claims to be 1.3 megapixels, it's actually only 300k pixels and doesn't work well in light, you'd be better off with a good high quality external webcam.