My co-worker Dan posted a blog yesterday entitled “How to Create an Alias Programmatically“. His example is 21 lines long, and is compatible with Leopard and earlier. It only works if the original item is a folder (though creating an alias to a file is not much different). The code messes with resource forks, and the C interface is generally difficult to read for someone who spends most of their time in Objective-C. The code is also not guaranteed to be 100% accurate, since Apple explicitly did not support programmatic alias creation in Leopard.
NSURL *src = [NSURL URLWithString:@"file:///Users/bjh/Desktop/temp.m"];
NSURL *dest = [NSURL URLWithString:@"file:///Users/bjh/Desktop/myalias"];
NSData *bookmarkData = [src bookmarkDataWithOptions:NSURLBookmarkCreationSuitableForBookmarkFile