Had the exact same problem 6 month ago. We implemented a solution from http://www.fastpasscorp.com/ called FastPass Password Manager, that was great - we have password synchronization now - that really was a relief for the ServiceDesk and for the users also. They did not require any new software on our AS400, but had a small transport for SAP. They have connectors for other systems also. We built integration to other custom systems as well using a Generic connector - all was quite easy to get up and running. About the installation - we actually paid them for the installation - think it took them half a day! We tried to implement SSO we did spend month trying to implement it and it never really worked. I think that the ?Single Password? approach was much easier - guess its like a 90% solution using 10% of the time...
Thats just my expirience.
\AMVIT