How is a DateTime object formatted into a String suitable for a String based SOQL query?

    DateTime dt = DateTime.now();
    String formattedDt = dt.format('yyyy-MM-dd\'T\'hh:mm:ss\'Z\'');

Update: Another option is to use SOQL dynamic binding, no formatting required.

    DateTime dt = DateTime.now();
    String query = 'SELECT Id FROM User WHERE  SystemModstamp > :dt '
    List<SObject> results = Database.query(query);