As seen from bug 5096, the clipboard monitoring system in Windows is very fragile and breaks if only a single application does something wrong. Fortunately a new API was introduced in Vista that does not suffer from this poor design.
We probably still need to be backwards compatible with older versions of Windows, so we'll have to implement this on a "if available" basis.
The new API is accessed using AddClipboardFormatListener().
There is also a report that if you start a Citrix client connection before ThinLinc it will prevent ThinLinc to get clipboard notifications until the Citrix client is closed. This is reported as Case 2 in issue15119 and might be related to that we use the old clipboard API for backward compability.