Multi-threaded software as a scalability answer is apparently harder than most people think. Consider:

With both SQL Server and Citrix Terminal Server installations, HT-enabled motherboards show markedly degraded performance under heavy load. Disabling HT restores expected levels, according to reports from within the IT industry.

I've recommended multiple processes vs. multiple threads for years, but most people figured I said that because VisualWorks doesn't map Smalltalk threads to platform threads. It's not just me though:

"It's ironic," said Ibbotson. "Intel had sold hyperthreading as something that gave performance gains to heavily threaded software. SQL Server is very thread-intensive, but it suffers. In fact, I've never seen performance improvement on server software with hyperthreading enabled. We recommend customers disable it when running Citrix and our software on the same server"

Of course, this doesn't help matters much either:

Earlier this year, Intel hyperthreading was revealed to have a security flaw where threads could find information from each other through the shared cache despite having no access to each other's memory space.

I'd be more interested in knowing whether that flaw can be accidentally exploited to cause problems in an application.