I had been keeping a very casual eye out for something pertaining to the HK45. The best I had found for months was a snippet from a German magazine article.
Until today. Found one over at the Air Warriors forum, a link to the Vickers Tactical reprint of a Shotgun News article provided in the title link. It is well worth a read, especially if you're a H&K groupie like myself.
Drool--over the article, not the forum. Although my father was Air Force, the branch of service I hold in highest regard are the Marines. But I digress.
There's another article floating around, according to the guys over at HKPRO, appearing in an issue of Guns & Weapons for Law Enforcement, by Ken Hacktahorn, who was instrumental in getting H&K to come up with the new design (along with Larry Vickers).
In short, the HK45 is very much like the USP45, from the modular action to the controls. The USP was a well-proven design, reliable even when fired dry (no lubricant, which unfortunately can be the case with service pistols), and so, only minor modifications were made, including some of the P2000's ergonomics, a slimmer profile, and I suspect a single-stack magazine (8/10 rounds, depending on whether or not it's for the compact or fullsize models, respectively).
Anyway, I'm writing all this in case you don't bother reading the article. It puts it a lot better than I ever could.