I think Socket AM3+ chipsets still require a separate northbridge and southbridge, which are impractical to squeeze into the mini-itx form factor. I found an AM3+ board on Google Shopping that was listed as mini-itx, but looking at the picture it was clearly micro-atx instead. On the Intel side, you won't find mini-itx boards for LGA1366 or LGA2011 either (though possibly for somewhat different reasons).