don’t know if this still applies, but here’s what I’ve used in the past:

Re-enable portable home directory options at login:

Mobile users in an OS X Server / Client setup are prompted at login with a dialog asking if they want to create a portable home directory on the machine they are logging on. They have three buttons to choose from:

1. Not Now — just log in to the network account and work with the network home directory. Question will reappear at next login.
2. Yes — creates a local copy of the network home directory and a special local user account labelled “mobile.”
3. Never — suppresses this dialog at login to the machine and continues login to the network account using the network home directory.

Up through OS X 10.4.2, “Never” meant anything but never, as clicking the “Never” button did not prevent the dialog from coming up each time the user logged in. 10.4.3 fixed the problem, and “Never” became really never. But what if you change your mind and want to finally create a portable home directory on a machine where you previously discarded this choice? There aren’t any controls to reset these settings, in either OS X Client or in OS X Server’s Workgroup manager.

The solution resides in a preference file on the client machine: /Library/Preferences/com.apple.MCX.plist

When clicking “Never” in the startup dialog, OS X creates this preferences file (or an entry in the file if it exists already). This file contains an entry for each mobile user having chosen to never create a portable home directory on the machine. The key looks like this:

Multiplications

The value is set to 1 for each ‘never’ user. After deleting the file, the dialog reappears at login, and users can successfully create a portable home directory on the machine.

In case there are more than one entry, I guess deleting the key/value pair concerning just one particular mobile user, without deleting the whole plist file, would have done it also, but I did not try that.