As far as scripting languages go, Lua is very fast, but like anything it varies depending on the processor. For example, Lua would not be great on a console platform because it tends to be very branchy, and some console platforms branch very slowly. To best answer your question, I would suggest running some benchmarks. See how fast Lua performs some various algorithms on the device. I suspect you'd be fine as long as you're not doing any heavy math or iteration in your script code. (You shouldn't be doing that anyway. They're scripts. Do calculations in the engine.)
That said, scripting isn't a silver bullet for platform independence. You may need to rewrite in Obj-C anyway if you port to iPhone due to Apple's restrictions on executing dynamic code. I don't have a link offhand, but it may be worth reading into. Apple barely allows managed code (you have to get special permission to run a special distribution). They most certainly will not allow a Lua implementation into the app store.
[edit] Apparently I was mistaken about the Lua on iphone thing. Take a look at the comments below.