The same thing happens with all new processors. They're likely using a DESKTOP processor (meaning it eats power like a man in the desert drinks water) and chipset and memory in a laptop which has to run on limited battery power. Though the site claims they're using Mobile processors, Intel has not introduced such a thing yet. The 845 chipset is also definitely a desktop chipset, not made for mobile use.
Dell and other makers are waiting for mobile processors. It's very slightly possible that those people actually have Mobile P4's and have simply jumped the gun and introduced products before Intel officially introduces the processors. They also claim to have 1.8 and 2GHz processors available. I really doubt that Intel is shipping out 1.5GHz AND 2GHz processors at the same time even were it in order to have vendor stock ready for the introduction.
[Edit]
Okay, looking at the Intel roadmap, they may actually be using the mobile version of the 845, but they don't specify it while they do specify mobile P4, which makes me think it's the desktop 845. The mobile 845 not being available may be what's keeping the big names from introducing systems right now, even if they are getting mobile P4's.
Given that the mobile P4 is supposedly being introduced soon (Intel's site shows only 1st half 2002), I suppose it's possible this company got a few 1.5GHz mobile P4's and is selling them before the launch. I wouldn't expect them to have too many though, the big names probably have a LOT if a small name has any (and I don't know if that small name would get more if they're jumping the gun by too long and piss of Intel). Those are certainly probably full Desktop 1.8GHz and 2GHz.