If there's one laptop that could be used instead of a desktop it's gotta be the 17-inch MacPro. That's the opinion in a review of the Mac at Computerworld.com.
The 2.6-GHz machine that the reviewer, Ryan Faas, reviewed weighs 6.6-lbs, a bit lighter than he expected. (Faas has always thought that a 17-inch laptop isn't a laptop at all, it's more like a desktop.)
The first item Faas details is the screen, one he thinks is the best in the world. Its native resolution is 1,920 by 1,200 pixels. (That's the upgraded resolution, which you pay a bit more to get.)
Second on Faas list of good things about the machine is its great gaming hardware--a Nvidia G3Force 8600M GT card. Gaming is good on this Mac.
Also worth mentioning is that Apple of late has made their MacBook pros faster than they did two years ago--50 to 75% faster. Faas tested the Mac to see if this was true, and many tests confirmed that it was.
Last to discuss here is the touchpad which "includes the ability to swipe through items (photos in iPhoto/Aperture, Web pages in Safari or songs in iTunes, for instance) with a three-finger gesture on the trackpad, as well as the ability to zoom the contents of an image or the text of a Web page by pinching, much like on the iPhone or iPod Touch."
If you have room for a big laptop or even want to use this laptop as a desktop, it's well worth the price.