The good news was that Rasmus and Nikolay couldn't see any reasons why PHP was particularly challenging. What we saw were general challenges in scaling applications to large traffic volumes - and that's pretty much where the panel discussion went. Here's a few key points from the discussion.
Highlighted points included: removing bottlenecks to help performance (simple in theory, hard in practice), the fact that PHP should never be the bottleneck, the "odd direction" of HipHop, challenges/growing pains in moving from startup to a large web property. There was also a less technical discussion about "scaling teams" and finding the right (motivated) people to do the job.