More or less as soon as Apple released the Ipad, Adobe released the free software, Photoshop Express. As the name, and price imply it is not even close to Photoshop in functionality, but it is a very useful tool to have available. Personally, I use it as something to facilitate micro-blogging of a photo shoot. During down periods I transfer images across to the iPad for review and to show the model, and if there are any that stand out as solid and not requiring much work beyond fairly basic tweaking, I can use PE to edit and then upload directly to Facebook.
A new addition, an add on pack, was recently released, Adobe's Blog. Mosly it offers functionality to the iPhone, including the ability to set a timer. Another pretty basic function that iPhone users have been making do without.
The key advantage for the photographer using their ipad is noise reduction. Now, honestly, I shoot with a 1D Mk III, and naturally at the high end of the ISO scale there is noise, but its limited, Fixing it is not something I'd trust to a £2.50 extension to an free app, when there are more sophisticated tools like Noise Ninja and now Lightroom kicking around.
What it does do is give a rather nice skin smoothing effect, effective for a very quick run-through when creating a working image. The filter doesn't seem terribly sophisticated when it comes to separating noise from small detail, which is a problem for fixing actual noise, but perfect for smoothing out imperfections in skin or clothes.