I think the property is called "PageChangedOnPublish". You could try searching for this in Reflector. I can see it's used in i.e EPiServer.dll in EPiServer.DataAccess.PageDB method SavePageVersionData
Maybe it can get you on the right track :)
@Piquet: When the mark page as changed is ticked the pagechanged date is updated.
And yes the subscription works as you describe in the latter case.
I haven't been able to determine exactly what happens when a page has the property "Mark Page as Changed" checked.
Does that page id get added to a db entry to be included in the next subscription run, or is it only that the property "PageChanged" is updated. If the latter case, does the subscription run therefore use the PageChanged property on a candidate subscription page to determine whether to include it in the current subscriptionJob run?
A pointer to where the 'Mark Page as Changed' property is handled in EPiServer dlls during a page publish would also be helpful so that I can "Reflect" it. I haven't been able to pin point it.