In the 2.5.1 shell a username is not required to do X509 auth:

$ ./mongo --ssl --sslPEMKeyFile jstests/libs/client.pem 
MongoDB shell version: 2.5.1
connecting to: test
> use $external
switched to db $external
> db.auth({mechanism: 'MONGODB-X509'})
1

A username should be required for a number of reasons:

1. It's a sanity check that the user is using the correct x.509 cert.
2. Not requiring the username is inconsistent with all other authentication methods, including GSSAPI which also doesn't technically require a username.
3. Not requiring the username will be inconsistent with drivers that have no good way to decode the cert and derive the username.