We are very grateful for the answer we just got from PTC Technical Support.
It turns out that the Method we are using to retrieve the file modification status (ie. the +sign from the WGM) and show it in an Attribute, from :
EPMWorkspaceHelper.manager.getUpdateCounter()
is not officially supported and so its behavior can change at anytime without notice - and indeed this is what changed from v9.1 to v10.0.
PTC recommend that we file an enhancement request to get a supported API that retrieve the modification status of an object in the Workspace, so this can be passed to an Attribute/Parameter, in much the same way that the following can be used to get the "NEW" flag (and although this Method isn't shown in the v10.0 M040 JavaDoc either, it is mentioned in many copies of the Customizers Guides and in TAN 150617) :
EPMWorkspaceHelper.manager.getNewObjects()
I have already posted an Idea for this issue : http://communities.ptc.com/ideas/1148, so if you land on this Discussion and like the idea, please follow this link and add your Vote.
In the meantime, the issue appears to be parked on CS129573 :
https://www.ptc.com/appserver/cs/view/solution.jsp?source=subscription&n=CS129573
Message was edited by: Nick Drought (01-Jun-2013) to add reference to CS129573