A Mac is a genuine Unix workstation that is much easier to administer, and has much better software and hardware support than Linux.
I can run basically every Linux/Unix application on my Mac, both command-line and GUI, while not having to worry about wireless networking drivers, printer support, power management / sleep support on my laptop, getting accelerated 3D drivers working, or any of the other minor hassles that are involved with setting up and maintaining a Linux install.
If you walk into the computer science department at MIT, basically all the faculty have a Mac, and fully half the students do. These people are not buying Macs because they saw a cool ad on the bus - they're buying them because a Mac is the best tool available.
The argument that Macs are just expensive, "designer" PCs that look pretty and sell well because Apple has marketed them well doesn't hold water. Yes, they have nice hardware, and a clean, polished, slick UI, and that does make them more pleasant to work with than some blob of Dell plastic running Vista - but they have the functionality to back up their appearance, as well.
Yeah, they're more expensive. If you value your time at all, you should realize that spending an extra $100 on a Mac is well worth it if it improves your productivity. Hell, if you ever spend two hours fighting with some weird issue on your Linux box, it's no longer saved you any money. You know how long I've spent fighting with the OS to get my wireless working, or hibernate working, or whatever, in Mac OS X, in the five years I've been using a Mac? Zero. I'm not exaggerating. It lives up to the hype. It "just works". It gets out of my way and lets me get things done.