Getting the current user's identity

So far in my web applications I've been connecting user-specific things together with the UserId from the aspnet_Users table. For example, say a user places an order - I'll get their UserId and insert this along with a new order into the Orders table. I get the current UserId like this: 
// get current user ID
MembershipUser myObject = Membership.GetUser();
string strUserID = myObject.ProviderUserKey.ToString();
Guid guidUserID = new Guid(strUserID);
However, I thought I'd try a different approach when it came to getting a list of orders made by a specific user - using an ObjectDataSource with a Parameter Source. So I configure my ObjectDataSource to use my OrderTableAdpater.GetOrdersByUserID method (I'm setting this up using the Configure Data Source wizard). However, the next page of the wizard apparently does not ...