Like the OP, my Apple ID (the one I use for all my purchases) is just a word. Let's say it's
abc. It happens to be the suffix of my MobileMe email address that is still active (
abc@me.com).
Since appleid.apple.com isn't loading on my computer, I just went to my iPad then went to Settings>>Store>>View Account. If you tap on your Apple ID it shows up the email address associated with your Apple ID. I just edited it to be my MobileMe email address. It didn't work the first few times and kept giving me an error but then a few minutes ago, it worked. I'm pretty sure you can change it in either iTunes, an iPhone, iPod Touch, or iPad, or appleid.apple.com (where you go to Manage Accounts).
So now, if I view my Apple ID, the email address associated with it says
abc@me.com.
If this doesn't work for others...maybe mine is a mistake =(