Since at least R2008b I've never observed an array, which is not aligned at 16 bytes. My Mex functions stop with an error otherwise and no user reported such an error yet. This concerns 32 and 64 bit versions of Matlab and Windows.
I agree that an official statement is required.
[EDITED] "Aligned at 32 bits" -> "Aligned at 16 bytes"
I've used SSE statements only, so I did not check for 32 byte alignment.