The iPad will arrive in Belgium in July 2010.
Unfortunately it’s not sure that the 3G will be possible with all operators :
“What is important to note here is that the 3G version of the iPad only support the SIM microcards (also called 3FF). This is a more compact version of the current 2FF SIM card which are used by all Belgian GSMs. To our [DataNews] knowledge, the Belgian SIM microcards are temporarily unavailable. Only the mobile operator Mobile Vikings have already announced microcards ordered from the manufacturer Gemalto SIM. They should be provided in June, just in time so before the launch of the iPad See http://mobilevikings.com/nl/blog/micro-sim-cards-ordered/ .“
(Translation of DataNews.be)