Just thought I’d update in-case anyone stumbles across this page in the future. As of 1.5.3, mongo now supports a real $or operator: http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-%24or
Your query of “(expires >= Now()) OR (expires IS NULL)” can now be rendered as:
{$or: [{expires: {$gte: new Date()}}, {expires: null}]}