I don't think anything official has been said but I can confirm that Service Desk 7.4 will happily integrate with LDMS SP3 without any patch required.
Just to add to my last response I've now put together a document to cover the LDMS version compatibility: Service Desk and LDMS Integration Compatibility Matrix (SD Data Import via LDMS connector).
We have a customer who self upgraded to SP3 when it came out and their DTM integration broke under 7.4. It wasnt until I went in there and upgraded them to ServiceDesk 7.5 recently that the integration returned. That said though I didnt do anything explicitly in this respect to DTM.
Have you had any other reports along these lines as you say it should/does work with 7.3/4 OK? I was wondering whether DLL's need to be re-registered or something, so interested if you had to perform any post SP3 steps in ServiceDesk so I am aware of for next time on another site.
Was the upgrade from 9 SP2 or an earlier version? It should have continued working if the upgrade was only from SP2 to SP3, perhaps the COM+ applications needed recreating - which the Service Desk installer does for you or you can be created via the LANDesk.ServiceDesk.Integration.exe program in the Console folder.
I wouldn't expect that to be required and in my tests it wasn't but without seeing the environment first hand we'll never know exactly what it was in this case. 7.4 and 7.5 are pretty much identical in their LDMS integration.