The "Apple Tax" is the higher price paid for Apple products over "comparable" products from the Windows side of the world (for some definition of "comparable"). It's usually worth it, because the Apple products are usually more capable than the corresponding Windows products. For example, my shiny new iMac not only runs Mac OS X programs, but it is already a fully-capable Unix box (including POSIX and X11), and now that Apple uses Intel chips, it is also a great Windows box. So I get the best of all possible worlds (even though one of them is Windows).
However, in some cases, it does come back to bite you. I was previously using a PowerMac G5 and a 17" Apple Studio Display. The two communicated by means of an Apple Display Connector (ADC), which looks like a standard DVI connector, but is different. This is the latest in a long line of "almost compatible" display connectors that Apple uses, which traces its lineage back to the old NeXT cubes, which also used a single cable for power, video, keyboard/mouse, etc.
My shiny new iMac has a built-in monitor, of course, but it also supports an external display through a new connector called a Mini DisplayPort. This is not really a proprietary connector like the ADC, but it doesn't appear to have wide-ranging support. So I headed down to the Apple Store to see what I would need to use my Studio Display with my iMac.
The answer is apparently: 1 Mini DisplayPort-to-DVI adaptor ($30) and 1 DVI-to-ADC convertor ($100!!!!).
So - for just $130, I can connect my several-year old monitor to my new iMac. Or - I could sell it along with my PowerMac (the going rate seems to be around ($50) and buy a completely new second monitor for not much more than the $130 price of the adapter.
Apple - you lose out on this price comparison easily.