After upgrading to an iPhone 3GS, I was pleased to see the new option to send photos and movies to MobileMe. I have two MobleMe accounts configured in Mail on my iPhone: my 'primary' MobileMe account, and a MobileMe sub ('family') account. The primary account is set to only sync Mail to my iPhone, and the family account is set to sync Mail, Contacts, Calendars and Bookmarks (since you're only allowed to fully sync/push one MobileMe account on the iPhone.)
The problem I found was that when trying to send a photo or a movie to MobileMe, the account that the iPhone insisted on using was my primary account. I searched settings and could not find anywhere to change this. I called Apple iPhone tech support, and although they understood the problem, they did not have a solution. (I called the day after the iPhone 3GS release, so they were understandably swamped!)
I just could not believe that the iPhone would not allow me to choose a different MobileMe account. Frustrated, I decided to find my own solution.
What I discovered is this: iPhone uses the first MobileMe account it finds on your phone when choosing which account to default to for sending Photos and Movies (regardless of whether it's a primary or family account.) As I had two MobileMe Mail accounts on the phone, and the first one listed in my Mail app was my primary account, the family account was being ignored.
My solution was this:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090626090011335