Since 2.5.0 you can now let user register with just an email address. They don't have to enter a username. A username will be generated upon registration and is not used anymore.
The generated user name is a the current timestamp minus 50 years. If you would register right now, that would be 51388621.
You can find the definition of this function here.
In a future version we will probably add a filter, so you can determine your own possible format.