How do I use a CCK field in a Rule to set the user role on registration?

Hi!

I have the following scenario:

I have a custom field in my user profile (a drop down selection) so that, when a new member joins up, they indicate whether they are a member of either Group A or Group B.

How do I program a triggered rule so that it takes this CCK value and adds a corresponding user role?

Ultimately, I then want to use role membership so that, when using Ubercart, they are presented with a custom price, depending if they are Group A or Group B. I can do this using the Price Per Role module. There will be other basic node privileges based on their membership.

I can see that the Rule Event needs to be when the "user account has been created" but when I add a Rule Condition I can't select the CCK fields. Instead, I am advised:

"The following conditions aren't available in this context because they require arguments that don't exist in your rule. If you want to use any of these conditions, you must first add some action that adds variables of this kind in a previous rule, or have an event that passes the required variables."

I can't see how adding an action is relevant here?

Is there another way around this?

Is it a case of coding an IF statement in some PHP code somewhere (where?!), i.e. IF CCK_Field='Group A' THEN role='Group A'?

Oh, what it would be to be clever!

Many thanks in advance of any advice offered :o)

Ali