As i understand it, .pch files are responsible for precompiling header files. Sounds like something that can rebuilt or recreated.
2 questions:
The .pch file is generally very simple, and its contents depends upon the project type. The basic idea is that it includes the heavyweight header files. Xcode itself doesn't modify this file beyond the basic template; it'd only be different from the template if you changed it yourself.
For a Cocoa application, it is:
//
// Prefix header for all source files of the '%s' target in the '%s' project
//
#ifdef __OBJC__
#import <Cocoa/Cocoa.h>
#endif
That's it! You can find all the templates in the SDK folder.
iOS: /Developer/Platforms/iPhoneOS.platform/Developer/Library/Xcode/Project Templates/