I don't use MobileMe, I don't want monthly charges nor do I want to store my data elsewhere. So needless to say I don't want a MobileMe icon on my finder bar, so I remove it. It's so easy, literally click and drag it off the menu bar:
You can also remove or reorder most of the icons in the top right finder bar in Mac OS X 10.5 Leopard in this way.
While you are at it, you may also want to remove the MobileMe System Preferences pane. No reason to look at extra stuff in your OS X system preferences either. To remove the offending Preference pane simply remove two files by entering the following command via the terminal.
sudo rm /System/Library/PreferencePanes/Mac.prefPane
sudo rm /System/Library/PreferencePanes/Internet.prefPane
Note: The sudo command is only for use by users who take responsibility for their own actions. These files may be replaced during future system updates.