"You need a message name (@selector), and the names are strongly typed -- meaning, they're pretty obvious with reflection tools. If there's a way to send a dynamic message without putting a @selector in your code, I don't know it, and I'm willing to learn."

sel_registerName() translates a C string to a SEL at runtime [1].

[1]: http://developer.apple.com/library/mac/#documentation/Cocoa/...