Doug G just because it has not happened it does not mean the server is any less unstable.
http://www.quepublishing.com/content/images/0789728494/webresources/A010701.html breifly comments on IIS's instability
also a recent search in the CERT Vulnerability Database yielded 28 hits for IIS vulnerabilities and while it is argued that apache has 25 but because of it's open source nature patches have been releaseed for a great many of these security holes. Now I'm not saying patches have not been released for IIS i am however stating that most patches for IIS are to fix it's instabilities.
http://searchsecurity.techtarget.com/generic/0,295582,sid14_gci1096044,00.html will give you a rundown of basic securing your IIS Server but people still get serious issues with IIS which Is why I like it for sandboxing only
and http://searchsecurity.techtarget.com/generic/0,295582,sid14_gci1093529,00.html will give you some information on a few of the most prevalent issues with IIS. Interesting they have all failed to mention that PHP isapi is much more unstable than php sapi for apache maybe it's shoddy programming and maybeit's IIS's instabilities showing through.
regardless of whether your Internet server is secure enough it may even be the case that it is more expensive than apache webhosting with similar features. With all this said it is my conclusion that IIS is all about getting less for your money