If you can't find a dedicated app (and it sure seems like something at least 20 people would have implemented by now) you could go old school and use safari and web sharing.
I have used the export to web page and export to slide show functionality of iPhoto/Aperture to save a html folder to the ~/Sites folder.
Then it's simple to turn on web sharing and use mobile safari to browse the thumbnail pages or just click through the main images as a slide show.
You can also export the slide show and a quicktime movie can be dumped if you would prefer better transitions and full screen with less tapping to navigate.
I might try out PixelSync next time I need to rate and do heavier editing from the couch or by handing the iPad to a client. Again - this won't be good unless you want your photos in a managed database as it won't work with just a folder of files.
And way out in "the OP didn't even ask this" land is a suggestion to check out PhotoMechanic - it stores all images in a folder but is optimized for extremely rapid optimized rating of many photos. There isn't an iPad/iPhoto link, but I've seen professionals blitz through 4.5k photos at the end of a major marathon and cull that first to 500 and then a second pass to the 35 images that make the day's news media picks for press use. With 12k images you might need a great tool on the computer to narrow down things before you move them to iOS land.