You could always make your own ePub from the official Wikipedia database download:
http://en.wikipedia.org/wiki/Wikiped...abase_download
Obviously there might be some issues with the dbase being so friggen huge:
'Current revisions only, no talk or user pages. (This is probably the one you want. WARNING: 5.6 GB compressed, up to 20 times that size uncompressed.)"
If you want to do this with Calibre, you might need to use the tools mentioned on the above page to cook the database into HTML.