I really like the <f32, ..4> syntax, but on the other hand, we may want to save special syntax for things that appear more often in normal code. The simd! one could just be a normal syntax extension, and then people could add more crazy types that they need. (MIPS accumulators comes to mind)

Unless we decide that cornering the dense linear algebra/DSP/GPGPU market is important for Rust, then really slick SIMD syntax might be worth it.