Well, writing as a scientist and a Mac-user, it has been my experience that some academics use Macs (usually pre-OS X!), but that most used Windows on their desktops; a small minority (those who were more into computing) used Linux on their desktops. The academics who used to run pre-OS X Macs take some persuading to switch to OS X and some have switched to Windows, unfortunately.
However, workstations used in labs, for instance, were often UNIX-based. UNIX workstations running Solaris and IRIX are commonly used for workstations running complex instrumentation, for instance. Use of UNIX is particularly common in Physics, where a higher proportion would use Linux on their desktops too. For stability for running long calculations (eg a week), multi-user access, and use of software like LaTex, UNIX was essentially the only choice.
As x86 hardware became faster and Linux became available, Linux became a cheaper option. Now that OS X is out, people are "curious." There is still some stigma about owning a "proper computer" but I know some computational chemists and physicists who are interested in Macs now, whereas they treated them with disdain before. Indeed, the UNIX guru for our campus (who happens to be the guru for just about everything, actually) in IT Services runs OS X on a Titanium PowerBook, and I've heard that many computer scientists at conferences like showing off OS X on laptops, whereas they used to run Linux... and show off that!
OS X is really what changed things, with the ability to run "normal" applications plus the UNIX/Linux apps they need. But then there was the issue of the speed of the hardware, and many were reluctant because they felt the G4s were not soooooo fast and that they paid over the odds. So Linux still won out for calculations and so on. With the G5 now, people are probably going to start looking again. I know one major figure who is increasingly interested, after what happened at Virginia Tech. So, I would say the future looks more promising for Macs in science than it did a few years ago, and OS X is the single biggest reason for this, and the old price/performance issue is the final perceived hurdle for many.