Data binding DateTime fields in MonoRail is pretty powerful and flexible, but not really straight forward.  Most of the time you can just provide the domain object propery as an argument to the a form helper method, but with DateTime's split across multiple select boxes this doesn't work when you post back to your controller method.  To get DateTime's to work across several HTML select fields you need to explicitly provide the selected value like so:

$Form.Select("billing.accountExpiration", $billing.accountExpiration.Month, [1..12], "%{tabindex='11'}")
$Form.Select("billing.accountExpiration", $billing.accountExpiration.Year, [$DateTime.Now.Year..2011], "%{tabindex='12'}")