It looks like Mac OS X is really starting to become a main contender in the OS marketplace. I think that OS X is roughly where Linux was 5 years ago; the 'techies' have got it installed and are busy playing with it, and are singing its praises. I think you only have to look for the proliferation of PowerBooks at the recent conferences to see that OS X is starting to become a home operating system for some people.
Of course, Apple has had a built-in advantage in that they already had a market share to start with (whereas Linux had to grow by itself) but there's probably a fair percentage of 'new' migrants to Apple, especially since some of the die-hard Mac fans are still slowly moving from the older System 9 (or 8) to Mac OS X.
Whilst Apple's foray into the server market is still a relatively recent idea, it's quite likely that it's going to increase exponentially, especially in markets where there's a large amount of data. Video comes to mind -- and it's no surprise that Pixar are migrating to a Mac OS X platform -- but with the recent release of Oracle 10g for Mac OS X, it's not difficult to imagine a time where server platforms are built around Mac units.
Although Macs have often been lambasted for their cost-per-unit when compared with other low-cost intel equivalents, Macs are built with the best features (USB, FireWire, SATA) whereas the low-cost units don't tend to perform as well. Comparing Macs really ought to compare like-for-like, such as with the higher-end IBM and Dell boxes. Importantly, when comparing with the server market, Apple systems are often as cost effective as their intel counterparts, but use a fraction of the power in doing so.
Lastly, Apple are innovating some cool technologies -- such as XSan and the XServe Raid, which will allow companies to easily put together multi-terabyte systems. This may not be of much interest to SMEs, but there are big users of data (especially in the video markets, where Apple already has a good market share) and big users of database software who will love these ideas.
I think that with the launch of Oracle 10g, Mac OS X as a server platform just got way better. It's not going to be that long before the likes of IBM start to migrate their platforms over -- they're already doing it with Eclipse -- and then we might see more Windows users converted to the Way of the Apple ...