Forgive me if I restate the obvious.
My understanding is that Sponsored Listings come first.
Then the Zeal/Looksmart Directory listings come next. I suspect that equally relevant directory serps may be ordered by how long they've been in their respective directory cats, with priority given to the older entries. This is a guess extrapolated from examining one of my client's serps.
Next come the Web Pages results, which I understand are pretty much Inktomi.
A couple weeks back, I noticed that Ink had set loose a lot of my unpaid listings onto MSN, which boosted my rankings to the top 7 (where previously I had been hovering around 12-17). I just rechecked and I've fallen back to my previous levels so I figure Ink must have re-crunched their DB to remove the unpaid listings of sites that also had paid listings. Darn!
I never mentioned this before because I didn't want to jinx my unpaid listings.
Could the expanded ink db have affected your rankings? Not sure if this is relevant to your situation but I think it adds to the general knowledge base and is food for further examination.