Correct, diaeresis (diëresis ;-)) or trema is used regularly in Dutch/Flemish, exactly as explained above.
It should only be used in case of ambiguity though. Eg. "reorganisatie" (reorganisation) has no trema, since "eo" can never form one sound, (unlike reverse "oe").
PS: what about zeeëend ;-)
(zee-eend = sea duck)
zeeëend looks awesome ;) We don't have the same issue in Norwegian, since we have differences in consonant length instead of wovel length, but that has it own set of problems :) That means that tak has a short consonant, which makes the wovel long, or takk where the consonant is long, and the wovel therefore short :p
I was two weeks in Heist op den Berg in a school exchange, and I have met a lot of great belgians and dutch people :) And Belgian fries with mayo, and all the beers, duvel, hoegaarden :) yummy!