I have a vague memory of the STL having trouble with aligned structs (e.g. SIMD vectors placed in a std::vector), unless you specify a custom allocator.
According to this document VS 2012/VC11 has partial support for c++ alignment. Does this mean that the VS STL implementation can handle aligned structs now, without providing a custom allocator?
No. It means that the VC++ compiler supports a method for specifying the required alignment for a type (the __declspec(align(N)) syntax). VC++ has always supported that, and it is basically listed as "partial" because "we have some alignment-related functionality, and it looks better than saying "not supported".
Apart from that, I'm not aware of anything in the C++11 alignment specification which indicates that SIMD vectors in a standard library container is guaranteed to work. C++11 alignment is basically just a formalization of what compilers already did, in this regard (as far as I know. I'd love if you could prove me wrong).
SIMD vectors are what the standard calls "over-aligned types" (see the part about "extended alignment"). What that means is basically "we guarantee nothing, and it's entirely up to the compiler how/if they handle such types.
In other words, implementing this part of C++11 would not necessarily change how SIMD objects are handled.