So, you know the extended support for XP ends in 2014, you know you're app will be supported for 10 years, and you're wondering about .NET 4.5 support for XP? What don't I get here?
Heck, if your project really is big, what are the chances it will be in production before XP is no longer in extended support? It seems to me like you're asking the wrong questions.
My project will also support Windows 7 (and probably 8). As my company moves operating systems in the coming years, my project will support it fine. But sadly, we have legacy apps that don't run well on Windows 7. So I am stuck for now. (Until 2014 probably.)
My project is slated for release about a year an a half. We will probably have parital releases before that. If XP is a no-go, I can't use .NET 4.5.