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I can see by the early postings that this will become a total Mac/PC flamefest as all these things come to be.
But to stay on topic, I think you will find the Mac to be a good development platform. Indeed, at my company, which is a Microsoft partner from way back, Macs are creeping into the development and QA ranks. They offer a very low level of upkeep and a very robust software design that doesn't crash. In addition, since they are based on FreeBSD, open source software and tools can be used if desired.
As far as using other Unix systems, Linux and BSD are used a lot at our company, but getting to a BSD box is far easier on a Mac with its built in software than the machinations that are required to do the same thing on a PC.
Usually people arguing against the Mac are doing so from a Microsoft based status quo, citing for instance, the Mac's poor support of the massive and overly complex network directory systems on Windows, or the cost issue, which rarely takes into account the return on investment. Macs don't need IT support in most cases which can also turn some IT personnel against them.