Web Analytics implementation challenge

The problem presented here should be generic enough for any solution provider. I will try to summarize the answers and provide them as a real business case for the Web Analytics Association and the University of British Columbia Award of Achievement in Web Analytics. Of course, I will also provide info on how I will end up implementing this!

Don't hesitate to ask for more info, this will just make the case better!

Here's the case:
While working on a web analytics implementation, we ran into quite an interesting challenge.

What would be your recommendation regarding this scenario if we want to track visitors across 3 (in fact, 4) different domains:

• SiteA.com (A) is the parent company, SiteB.com (B) and SiteC.com (C) are two different brand sites. SiteD.com (D) is a transactional web site. (B) and (C) share design similarities but are two different brand names. (D) serves the transactions for both (A), (B) and (C) with the appropriate brand facing.
• SiteA.com (A) refers traffic to SiteB.com (B) and SiteC.com (C). Those later two refers traffic to SiteD.com (D) for transactions.
• But sometimes, traffic goes straight from (A) to (D)
• So: A -> (BC) -> D and A -> D

The dilemma is this:

• We need tro track (B)+(D) or (C)+(D), that is, track the transactions with there respective brand sites. We do it on (D) by checking if the traffic is coming from (B) or (C) and assigning the right web analytics account id.
• (A) receives about 30 times more pages views a month than (B)+(C)+(D) altogether
• There is only about 10% of the pages on (A) leading to (B) or (C)
• (A) is actually conducting an evaluation for implementing a web analytics solution that might, or might not, be the same as (B), (C) and (D).
• (B) receives about 80% of the traffic referral from (A), the remaining 20% referrals goes to (C)

The questions are:

• Should we use a distinct account id for (A)? In this case, what are the compromises? For example, is there a way to track unique visitors across domains and/or conduct fall-out/funnels reports across different sites?
• Should we use (B) site id for site (A)? thus, merge (A)+(B)+(D)? What are the consequences? For example, this would significantly impact the ratio of visits to conversions!

Updates will be posted below:

• 2006-11-15: The joys of cross-domain tracking, on Lies, Damned Lies...
  Honestly, I think the proposed solution is a bit too complex and error prone. I would rather go for a non-intrusive solution such as a "friendly 3rd party cookie" outlined above instead of playing around with URL rewriting and introducing scripting code throughout the site.