We're in the middle of moving to Google Apps (I'll write on the experience when we're nearer the end). As part of the process we've taken the opportunity to change our email addresses to use the wycliffe.org.uk domain. So I've gone round various websites changing my email address. The process varies from simply changing a field once you've logged in, to a more complex flow involving verifying the new address. Some sites I couldn't change it, so I had to contact someone to do it by hand.
Some sites have the email address as the username, but you can't change it. I think this is just badly designed. It's not unusual for people to change emails, or for anything that might be work related, for a different person to take over a role. When the old email that hangs around is for someone who has died it just looks bad.
So when designing your site login system, consider how easy you want to make it for someone to change their email address.
I saw a post on a blog, I can't remember which, where someone had wanted to change their email, or retrieve a password on their own site. They were surprised how poor the experience was given the amount of effort they'd put into other parts of their site. Let me know if you've come across that post too and I'll link it here.