The answer to what happens to the photographs you have in MobileMe Gallery:
You will be able to continue using MobileMe Gallery through June 30, 2012, even after moving to iCloud. You should save copies of all photos published to your Gallery before that date. If you have a Mac, it is likely that most of your Gallery photos are already in iPhoto. If not, you can sync the Gallery photos to your iPhoto library. You can also download your photos and movies from the web.
In other words, it's gone. If you use MobileMe Gallery as an informal photo-sharing site, meaning like you would use Flickr or Facebook, then just move your photos over. But if, like me, you use it to send password protected photo sessions to models or clients and to back up your 5 star full resolution photos, you're out of luck. Time to start thinking about a different solution.
This is a shame. The MobileMe Gallery is a great product that's part of my workflow. It ties in with Aperture perfectly. I would imagine many photographers would feel the same.
The answer to merging multiple Apple ID accounts:
No. You cannot merge two accounts into one. However, you will be able to move your MobileMe account (firstname.lastname@example.org) to iCloud and, if you choose, you can continue to use a different iTunes account for store purchases and iTunes in the Cloud.
So, same as I'm doing today. MobileMe will become iCloud and iTunes will stay with an older Apple ID. I'd like to merge them, but this works.