After the conversion of userId to type long, and the introduction of a screenname it's very difficult to retrieve information from outside of Liferay (e.g. using SOAP).
In our company we use a central LDAP repository. We map the LDAP uid to Liferay screennames, but for example it is not possible to retrieve a user's roles using SOAP because RoleServiceSoap only provides getUserRoles(userId).
Storing the Liferay userId in LDAP is at most inconvenient (if not impossible) because we are using portal instances and the same user will have different userId in both, whereas we can assure he is using the same screen name.

A simple solutiion would be a service returning the userId for a given screenname.