Sure.
I know it didn't do it the first two or three times because I remember checking and thinking it was nuts that I couldn't change a user's details, but it's as simple as going to people and groups, selecting the user and choosing edit...
I don't know if something changed, but I remember looking at this a number of times and not getting anywhere. It was only after seeing my username at the top right of the page and clicking the dropdown to edit 'My Settings', that suddenly I was able to see email details to change them. Maybe there's something wrong with Sharepoint, maybe just there's something wrong with me. Don't know which one.
Would be interested to find out which. If anyone comes accros the issue that they can't see a user's email to edit it, try clicking on your username on the top right of the page and editing your own details from there. Then go back to the other users and see if you can now edit their emails. From what I remember this seemed tobe the scenario, but then maybe I am the one that's nuts, not Sharepoint. Please post your results here, so I know whether or not to check myself in to a clinic!
Paul.