Miguel's Mono gaming comments sparked a range of feedback on the matter, but this time rather than pushing games again, he's highlighting what happens when pushing Mono/.NET in the server-space. According to benchmarks by Phalanger, their .NET implementation of PHP is faster than PHP itself. Phalanger is a complete implementation of PHP that's re-written in C# and provides a PHP compiler and run-time.
Miguel's blog post from this morning is quite short and references updated benchmarks from the Phalanger project. From his post, "There are two cases on the language shootout where they are slower than PHP (out of eighteen cases) and they are also slower on eight of thirtyone microbenchmarks. But in general with real applications like WordPress and MediaWiki, the performance gains are impressive."
Seeing a derivative project outperform the upstream PHP project isn't a surprise. Facebook's been working on HipHop for PHP, which began as a PHP compiler to noticeably increase the performance of PHP. There's also the now-stale Roadsend project that hooks a PHP compiler into LLVM, among various other attempts to speed-up PHP. Facebook's HipHop or these other PHP alternatives were not tested in this comparison. At least, however, they did test the recently-released PHP 5.4 that does also offer up some performance enhancements.
The updated Phalanger PHP benchmarks in full can be found on this web-page. Besides increased performance, Phalanger does advertise its other advantages as being able to integrate with any .NET library (including those in C#, Visual Basic .NET, and F#(, and being able to use a "modern IDE" to develop code, a.k.a. Microsoft Visual Studio.