The ARM processor used by the iPhone/iPod touch has an extension called VFP for doing fast floating point operations.
If you just want to do a few floating point calculations you probably don't need to worry much. If you're doing a lot of them you should switch off the "Compile for Thumb" option in your project settings for maximum floating point performance. This will make your code run in ARM processor mode instead of Thumb mode. Thumb doesn't directly support floating point.
Here are a couple references that might help:
Break That Thumb For Best iPhone Performance
Floating iPhone | iCocoa
