Got this interesting info today on an issue with Enterprise Portal development.
When creating a new URL in the Web > Web Menu Items > URLs node in the AOT, you have to type in the relative URL to your SharePoint page in the URL property. There is an ellipsis lookup button that is supposed to allow you to lookup a page. However, when you click this button, you get the error message "Error in Url property error".
Apparently, this is not an AX issue, but there is Windows update to address this issue. You can find the download for the patch and more information on the Microsoft Support web site (KB907306).
After installing the update (I wasn't required to reboot the server), I restarted my Dynamics AX client and all was well when I clicked the ellipsis button to lookup a page on SharePoint.