Threadwatch member grnidone kicked off a beautiful discussion on cart abandonment and conversions a few days back, it's now ripe for your enjoyment:

She got the ball rolling with this little nugget:

As far as how we combat it, on one site, we place a cookie so when someone comes back to the site, they are offered a pop up window that tells the person they have something in their cart, and if they finish the purchase, they get 10% off their cart. That has worked well, we have a 13% conversion rate from that pop up alone. I think that could go up if we switch to dhtml to get around the pop up blockers out there.

So, what does matter when it comes to carts?