A lot of web designers want an impossible thing: their pages to look exactly the same, to the last pixel, to every user, no matter which platform or browser is he using. In many cases they try to circumvent this by using ugly hacks and Flash.
The fonts are one of the biggest cause and the lack of their availability is huge. I just started to address this on all my websites using the Liberation fonts pack.
It is very simple and obvious: you don't have any guarantee about a certain font is installed on your target system, but you may get a guarantee of having a metric equivalent of it, and Liberation is exactly this: the metric equivalents of the Microsoft fonts, so you set one font as the primary one and its metric equivalent as the fall-back option.
As Linux is my primary target, this is what I do in CSS:
font-family: "Liberation Sans", Arial, sans-serif;
Of course, is possible to do it the opposite way:
font-family: Arial, "Liberation Sans", sans-serif;