The major difference that I see these days between desktops and desktop replacement laptops, like the Mac Book Pro, is the ability to upgrade. With a desktop it's very easy to upgrade components like video cards and add multiple hard drives that are far less expensive than the 2.5" SATA hard drives used by the new line of Mac laptops; the iMac is an exception to this general rule because of it's all-in-one design that makes multiple internal hard drives not an option.
Though some will argue that you can get an external hard drive that's just as fast as an internal, I think you'll be hard pressed to find a quality external drive for the same price as an internal drive of equal capacity. Also, I've heard mixed opinions on external drives and whether or not you should leave them on continuously, as you probably would if you were using your laptop as a desktop.
But with the availability of 120-160 GB internal notebook hard drives, a Mac Book Pro with the 256 MB video card option would make a nice desktop replacement. The only time you'll probably need more storage before higher capacity notebook hard drives arrive, is if you do a lot of high quality digital photography or video editing.
I do agree that having both is still better. You could buy a Mac Book and a 20" refurb iMac for about the same price as a 17" Mac Book pro with ram and hard drive upgrades, or a 15" with upgrades and an external monitor, keyboard, and mouse.
I'd still wait a few weeks on making a purchase though because the WWDC is coming up, and Apple always has new goodies to show off there. Rumors are around that Apple will upgrade the chip in the Mac Book Pro to Intel's Core Duo 2 mobile processor, which would make the Mac Book Pro more powerful than the current Core Duo iMac. This introduction could also cause a price drop in Core Duo models. We'll have to wait and see though.