Is it bad practice to use unicode symbols or shapes in a app?
I don't see anything really wrong with this shortcut approach, especially given Apple's concern for typographic quality. In your shoes, I would consult the Unicode Code Charts, and make sure I'm very carefully specifying a programmatic unicode character rather than relying on typing it in my editor.
Since cocoa uses unicode extensively internally, and since most API methods that don't specify encoding have been deprecated for the last couple iterations of OS X, I think you're pretty safe. As you're writing a desktop/iphone app, rather than a webapp where the deployed fonts are unknown, you should be OK from a bitmap rendering standpoint if you stick to unicode characters that can be rendered by the known default fonts that ship as part of the system.
As long as you ensure it's one of the symbols included in a standard system font and it's set in the right font, there shouldn't be anything to worry about. Apple itself uses Lucida Unicode symbols all over Aqua. The only way that could go wrong on an end user's computer is if his system was broken anyway.
No; Cocoa has strong Unicode-fu. If you do have a problem, it'll probably be your fault—most likely, converting to or from the wrong encoding. (GCC used to have problems with \u sequences in @"" literals, but I believe that's fixed now.)
On the other hand, using characters such as on a webpage is a good way to confuse non-Mac users.