Currently using KMail 4.7.2, for OpenSUSE 12.1 Workstations and LTSP Thin Clients.

All previously configured to POP3 accounts using a local mail server, called 'mail'.

Have now attempted to replace POP3 options with IMAP. Workstations are OK with no issues. However, whilst Thin Clients appear to initially configure OK and display incoming mail in the 'IMAP Account Inbox', on checking I note that no new mail is displayed (despite selecting 'enable internal mail checking' every 5 minutes), and if I re-check IMAP Account settings, then all Thin Clients complain: 'Unable to connect to the server, please verify server address'. This problem occurs whether I define server as 'mail' or by it's IP address.

Are you trying to have multiple people signed in to the same account at the same time?
Also, can you verify that the IMAP server is fully reachable from the Thin clients? There should be no difference between Thin and Fat clients here...