It sounds incredible, but it’s true: OGC Filter Encoding in it’s current version 1.1 lacks an IN operator [1]. Combining multiple PropertyIsEqualTo using ORs performs really badly both on Arcgis Server [2] and Geoserver [3]. That calls for the implementation of this standard-exceeding IN operator. Maybe PropertyIsIn?

[1] http://www.opengeospatial.org/standards/filter
[2] http://viswaug.wordpress.com/2009/01/20/filter-encoding-standard-11-and-the-curious-case-of-the-missing-in-operator/
[3] https://underdark.wordpress.com/2010/06/10/dynamic-styling-and-filtering-of-a-geoserver-wms-using-openlayers-layer-wms-post/