I finally soughted out what is causing the problem -
it's Vista's UAC!!
When modifying or replacing a file somewhere below the "program files" directory without an installer, Vista makes a shadow copy of the file in the "virtualstore" and let applications that have used this file work with this "old" version. If you search your PC for files, windows search will not list files from within this virtual store.
That's why the strategy could still compile, even after I had removed ALL instances of the jar-file from the pc that I could find through file searches in explorer.
After I got a clue what was happening, I was able to find the shadow copy in the virtual store:
C:\Users\<username>\AppData\Local\VirtualStore\Program Files\Java\jre1.6.0_07\lib\ext\...jar
After deleting this file, the strategy compiled with the new version of the jar in the jre's original lib\ext dir.
Therefore as a general rule in Vista it is somewhat dangerous to manually replace or modify files below "program files" and should be avoided if possible!!
If there's no other way, the virtual store should always be checked for shadow copies to go sure the the applications are using the modified/new files.
After this experience, that took me quite a while to figure out what was going on, I
strongly agree to use the @Library annotation to point to the jars instead of placing the files in the lib\ext dir.
Thanks again for your thoughts and help!