For ASP on non-Windows platforms, take a look at ChilliSoft's attempts.
www.chillisoft.com - I've not used it, but I believe it has very good reviews and works very well.
On the why run ASP on OS-X angle, I'd have to ask the same question. As nice an OS as OSX is, it's really not a large scale web hosting server OS. And I doubt you'll see development for ASP support coming quickly, esspecially when OS's like Solaris, Linux etc etc have already got it and are probably much better suited to hosting mass numbers of web sites.
That aside, it'd be nice to be able to TEST an ASP site,
on your dev box right? Honestly, your ASP test box should really be a true NT/IIS/ASP box, and the arguement that you can't tell your clients that they can't run ASP, is indeed a difficult one.
For cost and performance sake, I would seriously be looking at an x86 machine, running Solaris, and some form of existing ASP support. Save the Mac and OSX for the workstation where you can enjoy using it
*shrug* then again.. I totally understand your position.
Good luck, Andrew