While I use an IDE for most development, I've been just using an editor for editing ARM asm on the Mac. Originally I was using Vi, but everyone's raving about Sublime Text 2, so I'm giving that a try. It does syntax highlighting (something you miss when you're used to it), but doesn't support ARM asm out of the box. It *does* support TextMate language bundles, though. Here's one for ARM asm:

 https://github.com/aumuell/ARM-assembly.tmbundle

I know Vi will also have good support for ARM syntax highlighting, but my Vi-fu isn't strong enough to pursue that.