Contrary to the descriptions of the parameters on http://www.mediawiki.org/wiki/Parameters_to_Special:Export, Special:Export does not accept parameters limit, dir and offset.
When I try to get xml of limited number of revisions of specific wikipedia page, Special:Export always sends back all the revisions in ascending order. This cause malfunctions of receiving script due to the size of the xml.
As far as I know, this trouble appeared 25 or 26 March 2009. I was running some scripts to obtain past revisions of wikipedia pages and until then they had no problem.