How can my iphone app detect its own version number?

As I describe here, I use a script to rewrite a header file with my current Subversion revision number. That revision number is stored in the kRevisionNumber constant. I can then access the version and revision number using something similar to the following:

[NSString stringWithFormat:@"Version %@ (%@)", [[[NSBundle mainBundle] infoDictionary] objectForKey:@"CFBundleVersion"], kRevisionNumber]

which will create a string of the format "Version 1.0 (51)".

Building on Brad Larson's answer, if you have major and minor version info stored in the info plist (as I did on a particular project), this worked well for me:

- (NSString *)appNameAndVersionNumberDisplayString {
    NSDictionary *infoDictionary = [[NSBundle mainBundle] infoDictionary];
    NSString *appDisplayName = [infoDictionary objectForKey:@"CFBundleDisplayName"];
    NSString *majorVersion = [infoDictionary objectForKey:@"CFBundleShortVersionString"];
    NSString *minorVersion = [infoDictionary objectForKey:@"CFBundleVersion"];

    return [NSString stringWithFormat:@"%@, Version %@ (%@)", 
                appDisplayName, majorVersion, minorVersion];
}

Now revving a minor version manually can be a pain, and so using a source repository revision number trick is ideal. If you've not tied that in (as I hadn't), the above snippet can be useful. It also pulls out the app's display name.