%LOGONSERVER% is defined during the logon process and is set to the %ComputerName% of the validating Domain Controller. Prior to Service Pack 3, it was not available after the logon process unless a SETX was used to create new environment variable.
User Manager allows you to set a users profile path using environment variables:
%logonserver%\<share name>\<profile>.<man or user>
Simply use directory replication to insure the profile is available on all Domain Controllers.
NOTE: When a user logs off, an updated copy of their profile (non-mandatory) is stored on the %LOGONSERVER%, so unless you implement fast replication, don't use this technique with regular roaming profiles, because it is possible that the user's next logon will use a different %LOGONSERVER%.
This technique is specially valuable when your network is hosted on a WAN and each office has a local BDC and you are using a LMHOSTS file to insure that the local BDC is the %LOGONSERVER%.