Actually, I don't think you have to do this because when you build your project, you choose what firmware it is targetted to, like pgb mentioned. Then you can simply use #ifdef
directives. Here is more information: http://stackoverflow.com/questions/845733/iphone-check-firmware-version A much more thorough answer is posted here: http://stackoverflow.com/questions/820142/how-to-target-a-specific-iphone-version
EDIT: I removed some text after realizing that systemVersion is actually a string, source: UIDevice Docs.
If you will use it in various files across your project then you can maybe organize it by adding it as a property to your appdelegate, and them retrieving the value by doing something like this:
MyAppDelegate *theDelegate = (MyAppDelegate *)[[UIApplication sharedApplication] delegate];
NSString *version = theDelegate.version;
But that might be too convoluted for something so simple as the version number.