I'm employed (in the UK) mostly writing PHP on Apache, but I have also been asked to work with ASP/IIS a number of times, so I can only echo what has been said already: Learn both.
Generally speaking, I've found that IIS/ASP is mostly used by in-house developers, where a company develops and hosts their own sites, whereas PHP/Apache is more commonly used where the company outsources their websites to a specialist ISP or development company.
Why is this? From what I've seen, it's simply that most companies who decide to develop their websites in-house already have an IT department with strong Microsoft skills, so they pick the solution they have the skills to support.
On the other hand, the specialist ISP has the skills to support whatever platform is required, but most of their customers don't actually care what platform they use, so they pick the one that is most cost effective or preferred by their development team, which usually means PHP.