We know that there will be dual and triple channel DDR3 Nehalems targeted at the desktop market and I've also heard about quad channel (FB-DIMM) chips for the server market.
With all these variations it is uncertain whether Intel will use a single socket for the lot of them. Maybe they will use a single socket for the desktop parts (LGA 1366) and another for the server parts but nothing is certain yet. With the IMCs being so different the pin count for dual,triple and quad channel chips will vary a lot.
I do not know if we are going to see desktop mobos with 3 DIMM slots but we are definitely going to see boards with 6 DIMM slots like Foxconn's
Renaissance mobo.