Actually, I'm Belgian, live in Belgium, and work in the Netherlands.
I always had problems when searching for a name with a 'ij' in our filing system...
In fact, in Belgium, we learn(ed) that the alphabet ends 'x ypsilon z'. For us, y and ij are something different.
When we learned to write capitals, we wrote 'ijs' at the beginning of a sentence as 'Ijs', not as 'IJs'. What we learned about the 'ij' was, like Horlings says: 'Eigenlijk bestaat de ij niet eens. Het is een samengesteld letterteken' (As a matter of facts, the ij doesn't really exist. It is a compound lettersign.') But in Belgium, we still consider it as a compound lettersign, just like (other) diphtongs like ei and aai and ieu. So accordingly, we sort ij and y different.
It's always fun to see how the same language is treated just a little bit different on both sides of the border.