I dont think you will be able to "pre-Order" vista, I think alot of shops have "jumped the gun" on this one, since I believe Microsoft are doing a "pay for use" policy.. IE, during the install you are asked what your most likely going to use it for, and what component programs you wish to install... then your billed on what you have installed, instead of paying £75 for an OS and not using half the features. In a way its a good Idea.... but dont quote me, its just somthing I heard.
Windows XP sp2 was probably the best OS microsoft released, yea it had its ups and downs, but they kept on top of patches and the OS virtually repaired itself with "Microsoft Updates". Such a versatile OS, Microsoft did well here...
I think Vista needs to sort out its DirectX to be compatable with more older games. I have been testing it for almost a year now, virtually all the drivers are no longer beta, and it still wont play games like "Black and White 2" or "Oblivion", there is some serious Vector Bleeding in both ATI and nVidia GPUs when I tested these games. Its a fantastic OS, it feels nice and smooth to use, just thr right amount of eye candy... BUT it flaws on probably its most biggest market for home users - Games.
I would 'predict' that On Vista's release, nerdies will probably purchase it, hackers will crack it, people may be forced into it by retail stores selling new PC's, but home users and businesses won't touch it till atleased the first SP release, based on microsofts reputation on past OS's.