Ok, apparently iDisk won't even download a file if it doesn't support the type. Very annoying. I didn't know it had that limitation. Sorry for sending you on a goose chase.
The best alternative I can think of is GoodReader. It's worth every penny of the $4.99 price tag. It's the closest thing the iPad has to a file management tool and very handy. You'll see it mentioned and recommended all over the forum
I did test Photoraw Lite with GoodReader and was able to download a RAW file from iDisk and then open it in Photoraw Lite. It was free so I picked it up and experimented.
When/if you grab GoodReader, go to the list on the right and choose Connect to Servers. You can add your iDisk account here. Once you do that you can choose the iDisk to browse for both downloads and uploads.
Once you've downloaded a RAW file, you'll be tempted to tap on it and try to open it. Won't work. GoodReader doesn't support the viewing of RAW files, though it will work with most other common file types. Despair not, you can still open it in PhotoRAW.
Select Manage Files from the list on the right. When you do this you'll see a circle appear on the left of the file name. Tap on the file to select it, and the circle fills in. Now chose the ••• Open In... button from the control panel at the right. You'll see a list of compatible apps, and PhotoRaw Lite should be there. Just tap on it and PhotoRAW Lite will open and start processing the file.
Personally I found PhotoRAW Lite fairly useless. It displays the photo (and you can only load one) with a few sharpening filter options, but that's it. I've been keeping my eye on the full version, and it promises better functions, but I notice that the reviews in the App Store are very low. For $9.99 I'd wait to see if they mange to work the bugs out.
I've also been watching PiRAWnha and considering. It has better reviews, though neither apps has enough reviews to inspire confidence.
In the end I'm waiting for my iPad 2 before trying my iPad as a photo editor. I'm on a waiting list, and probably have at least a month to think over my options.