When Mobipocket Creator is reading HTML files, it treats the tag:
<mbp

agebreak />
as a page break.
I don't know whether Calibre does the same.
Darn it, that didn't look right - it converted colon p into a smiley. That should be mbp, colon, pagebreak
See
http://www.mobipocket.com/dev/articl...agref_mobi.xml for mobi custom tags.