Bjarne,
I would suggest opening your Eclipse log file (workspace dir\.metadata\.log in a good editor like Textpad or UltraEdit, basically not notepad) then open a JSP file in MyEclipse like you normally do, then open dreamweaver, edit the same file, go back to MyEclipse and cause the autocomplete to fail… do you see a log entry added to the log file when you did that? Does MyEclipse ask you to reload the file due to contents changing on disk?