Where google analytics is set up for cross domain tracking (e.g. domain.com and secure cart.otherdomain.com) the _setAllowLinker is required.
However, the GCE plugin won't allow the required
_udn = "domain.com";
to be saved above the control code. (it just plain won't save it to the database). Thus, it's not possible to begin experiments as there is always a "Web server rejects utm_expid." error.
See for example,