Okay there is something weird going on. I'm not sure if this is the API or a inconsistency in the database. I want to get the first revision (and optionally n revisions after that). This does work with the The Big Bang Theory article titles=The Big Bang Theory&rvendid=145491640. The rvendid is not the ID of the first revision but some revision I choose randomly. It does return me the first revision visible by the parentid.
Now this is a different story on the Main Page titles=Main Page&rvendid=139871. It now returns directly that revision which might be because it's parentid is higher than it's current id. Even http://en.wikipedia.org/w/api.php?action=query&prop=revisions&rvprop=ids%7Ctimestamp&titles=Main%20Page&rvstartid=139890&rvdir=older&rvlimit=50 this is just returning two revisions.