There have been a few times where I've used unicode symbols in place of small icons in one of my Cocoa apps, either because it's easier to draw inline with text or because I didn't feel like firing up Photoshop to draw a simple arrow. I've wondered though, could there be issues with localization or fonts I might not be aware of? Are there any cases where these symbols might not match what I'm seeing on my workstation?
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.
I wouldn't do that. not consistent with the look and feel you usually get on an iPhone or a mac...