My personal opinion is that you can'y go too wrong with a Mac. Especially now that more hardware seems to be supported than in the past and you can run Windows XP on the Mac anyway. If you have some sort of software or hardware that you need and you can't get it to run on the Mac it will run in Windows XP, right?
The Mac OS X has much better support for hardware like printers and scanners than Linux OSs.
We have to wait and see what Windows Vista will be like. It could be great or it could be a dud. One thing for sure, it looks like Vista will require a powerful computer (unless they can make Vista more efficient). If you need a very powerful computer to run Vista that sort of shorts out the argument that Macintosh Computers are too expensive.
There are real concerns with Vista about software/hardware compatibility. You can bet the older stuff probably will not be updated. Mac OS X may have more software/hardware compatibility than Vista when Vista first comes out. As far as I have been able to find out there are issues with Vista when it comes to Photoshop Elements 4, Photoshop CS2, various antivirus programs and antispyware programs, some Epson scanners, etc. You may have to buy new software and hardware designed especially for Vista.
Better hope you don't need to reinstall Vista too often. I think right now you can do exactly one reinstall, unless that has been changed.
We really will not know if security in Vista is better until after the product comes out. There are some indications that security will not be that much greater than Windows XP.
So, in conclusion, if Vista requires a very powerful computer that Mac suddenly does not seem so expensive. And the Mac already has good security. And the Mac may have better compatibility with software/hardware. After all, you can run Windows XP on the Mac!
A Mac would make most people happy except for heavy duty gamers. And they can buy a game console.