I've got all of our IT staff and a number of the developers working on the server issues we had over the weekend and today.
We recently added additional webservers running Windows 2003 Server. It looks like there are some known problems with running 2003 server and the session state server. These problems only come up during extreamly high usage times and thus we first noticed them on Sunday.
The different areas of problems pretty much all relate to this one session issue. For example, I've seen a number of reports of can't upload images. The root problem here is the same as not being able to view pages or stay signed into the site as a whole.
To make things even more interesting, eBay enabled us for Checkout 2.0 today. Which means that combined checkout is now live. We have worked closed with eBay in turning Checkout 2.0 on and are continuing to monitor the site to see if any errors are coming in specifically related to combined checkout.
If I feel the site won't be in a more stable condition within the next 90 minutes, I will put up a site wide alert letting people know.