No doubt you found and followed some tutorials to change your admin user name, you most likely found some old MU tutorial and then found that your new WordPress 3.1 Network Admin menu/area just disappeared.
Well this quick set of pointers should help you out! 
Log into your PHPMyAdmin or what ever you are using to edit your database.
1. Find the table “_users”.
2. Edit the row where the “user_login” column is equal to “admin”. Change the admin part to what ever is your desired user name.
3. Also edit the row where “user_nicename” column is equal to “admin” (same as above) Change this to your desired “nice” name (no spaces or such) – This is also used within BuddyPress for the profile link and @admin.
4. BuddyPress users will want to change their profile where it echoes out “admin”, this is located in the table “_bp_xprofile_data”.
Now you will most doubt have done all that but now can’t find the “Network Admin” link and section, well you need to edit one more thing:
5. Open the table “_sitemeta” There is a row “site_admins” with data under the column “meta-data”, it will look like this:
a:1:{i:0;s:5:"admin";}
You will want to change this including the number 5 which is the character count of your username, so in my instance it will look like this:
a:1:{i:0;s:3:"tim";}
Save and test it out, that should fix the issue for you.
Please note: There could well be other instances in the database on a multisite install, I haven’t thoroughly checked. I only worked through this to help another user find a solution, this worked for me and then worked for them. Always make a backup of your database first!
Hopefully this should help someone out there! 
