It's not necessarily "scaling" in general that's a problem...I should have used a better word. Scaling, especially w/r/t JSP/Servlet containers, generally means scaling for load/# of requests.
That's not the problem. The problem is how many different sites run by different users a given container can handle.
Because JSP/Servlets are something that is not generally used in the shared hosting world where hundreds of strangers share a machine, none of the container developers have focused on making their software work well in our situation.
JServ does work well, (the biggest thing being the sandboxed shared JVM), but nothing since it has tried to replicate the feature (mostly because not very many people care, and it's a huge PITA).
I talked to the Resin people way back when, and like Resin, but they told me it wouldn't work well in our environment. And JRun is prohibitavely expensive (anyone who would pay that much to use it in shared hosting won't be using shared hosting).
That's pretty much where things lie. I've not investigated this stuff in a long time, but I haven't heard anything to the contrary since...