I'm using a 64gb iPhone for this reason. If you're spending the cash, an extra $100 to double your memory (from 32 to 64gb) seems like a decent investment. I had a 32gb phone and was running out of room, granted once I saved all my camera roll pics on the computer, I freed up quite a bit of space.
For the AT, you can look at getting an online photo service (e.g. photobucket, Flickr - both of these have apps) and upload your pics when in town (I would do this in wifi and not 3G though). If you start running out of room on the mini, you can just delete them since you will already have them online. I would use a service I can upload and download full size pics preferably. Most of these also allow you to upload video, but Vimeo and Youtube both have their own apps that allow for direct uploading to their sites (if you are using either one). Again you can delete the videos off your device to save memory (even with a 64gb size, I suspect this will be required). Doing it this way allows you to buy a 32gb iPad for instance and in theory you could put that extra $ towards a storage plan with one of these photo services (e.g. photobucket offers 2gb free and extra storage starts at $30/yr for 20gb).
Waterproofing - you can look at getting something like a Lifeproof case, they don't seem to have an IPM yet but I'm sure it won't be long. You can also buy/make a waterproof cuben fiber sack and use that as far as storage. I would probably go the CF route myself if not using the device while hiking (for photo and video taking). It's significantly lighter than a real case. Personally I wouldn't worry about the glass, other than the weight itself. As always HYOH.
Dropbox is great but the downside is that the dropbox folder on my 1 computer is 1.5gb, and on my other computer that same dropbox folder is also 1.5gb, on a 3rd computer, that folder is also 1.5gb. So now I have 1.5gb, taking up 4.5gb of space across all my computers. As far as the app on an iDevice goes, it physically contains only your dropbox favourites on the device itself, you can see all of your files of course. Of course you're free to download everything if you'd like, but the limitations is still the free account storage limit or you pay. There are ways to get more free storage, I'm up to 6.2gb free so far (one way was to upload I think it was 2gb of photos and you got to keep that 2gb, I deleted the photos a day later to free up the space).