I currently use BeerSmith for the PC -- and the version of BeerSmith for the iPhone is for view recipes, and making on the spot calculations. So, I have not yet gone that route, but I will probably add the iPad version of BeerSmith once it become available.
However, I do have a couple of iPhone Apps that are brewing related.
BrewMath -- this allows me to calculate Strike Water Temps, Mash Out Temps, Hydrometer Adjustments for temperature, conversion for SG from Brix when using a refractometer and a lot of other tools. It does not have recipe formulation, but it does allow you to do a lot of on-the-spot calculations for all grain brewing.
GetHoppy - This is a database of different types of hops. I put it on my iPhone specifically so that I can use it for when I go the LHBS. It gives a lot of information about different hop varieties. I use it for hop substitutions. It also has a database for Fermentis, White Labs, and Wyest yeast strains. Finally, it has calculators for determining how many ounces of a partiuclar variety of hop to substitute for another. It presumes that you want a certain bitterness and you can calculate how many ounces are needed of the substitute hop to achieve the same bitterness for a given mass of the original hop. There are also calculators for Refractometers and Specific Gravity Temperature Corrections.
I am sure that this is not what you had in mind, but I find them helpful, especially for when I am at the LHBS and need some information.
Mark