Max Load?

I wonder if anyone has put WordPress to a big stress test in so far as the number of users and the number of posts it handles. For example, would something like 1000 or 10,000 posts a day from as many users somehow overburden the system? Any potential problems with simultaneous posts from many different users? Any potential problem with having such posts blasted in over the XML-RPC interface?

I understand there’s a matter of hardware (CPU, memory, etc.) But outside of that, how much can this software really endure in terms in large influx of simultaneous incoming data?

I know for example Blogsome uses it to host somewhere around 4 or 5 thousand blogs. That’s a big load. But how much can it really take? What if all 5 thousand users were posting to the same blog and half of them were posting at the same time? Any ideas how that might work out?