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).
Try LogMeIn on the iPad. It just takes over the screen of your MacBook. So, you just use your Lightroom app on the iPad. You connect via WiFi.
The app is free. I found it a bit clumsy because of the lack of an easy mouse-click and too slow.
The built-in Photos app on the iPad writes XMP. I've used it to crop & straighten RAW files before syncing into Lightroom and continuing there. Unfortunately the UI is not exactly on par with e.g. iPhoto for iPad, and I'm also looking for better options. Having the iPhoto UI for basic editing into sidecar files would really make my day.
This question has been overcome by events (OBE): Adobe has released Lightroom Mobile for iOS and Android.