phommel
Refugee
That seems to be the likely reason, yeah.
Is there a suitably sized Dutch stadium free at this time of the year? I'm surprised not to see a date there too, especially since I assumed the presence of only 2 Amsterdam dates this year instead of 3 to be a sure sign the band would return in 2010. Plenty of room to slot in a couple in September though ...
That's a good question. The stadiums I can think of that are large enough are Feijenoord Rotterdam or Amsterdam Arena. However, every other weekend those stadiums are occupied for a soccer match. During the week there is Europe League or Champions League... All U2 stadium Tours so far visited Netherlands in July/ early August, with the exception of Zooropa (May, but end of soccer competition)
Thinking of the long period the stadium is needed for building up / breaking down, I think it's not impossible, but it will be difficult to plan Dutch concerts.
Besides the big september gaps, it might also be possible that they will plan dates before Frankfurt...