App to replace some of my acronyms
The most prominent app would by TextExpander, available for Mac OS and iOS. Another alternative would be Typinator only Mac OS.

Another option (on iOS 5) would be shortcuts (Settings -> General -> Keyboard). You can add shortcuts and replacements as in your example. This is probably the best solutions as it is free and works in all apps. (TextExpander works only in some, that explicitly support it.)