iPad photo editing app that exports edits as XMP to reuse in lightroom? Basically, I'm looking for Lightroom for iPad :) but until then, I'm trying to find an alternative.  

To organize and tag pictures I'll be using Photosmith which integrates with Lightroom. 

Now, I need an app to do basic editing of pictures (cropping, exposure, saturation). But I need an app will do this in a non-destructive way and that will make the edits available as XMP so that later, I can just copy the XMP as sidecar files and load them in Lightroom (or better, it could load them via a lightroom plug-in the same way Photosmith works).

PS: I posted the same question on photo.stackexchange.com, see here.
Have you tried Adobe Revel?

Basically you can edit photos on your iPad, store them on your computer and the edit them there as well.

It's a subscription service with many of the features of Lightroom.