Sometimes, and for a seemingly random selection of users, SharePoint will display a user as Domain/User rather than using the Preferred display name. So for example instead of Displaying my name as Neil Cummings, it will show as SharePoint/ncummings.
One way to resolve this is to reset the sync DB by running the following commands:
stsadm -o sync -listolddatabases 0
stsadm -o sync -deleteolddatabases 0
stsadm -o sync -synctiming m:1
stsadm -o sync
This is non-destructive and may resolve the problem.