i have just visited my local montessori nursery school with a view to sending DS1 there once he is 3. it was very impressive and all the children were a bit freakily well behaved, but it was just far too ordered for my liking. i think when they are small and meant to be 'leaning through play' that a bit of chaos and plastic toys (it was all very puritan and spartan in its layout and equipment) are what he needs at this age.
it might help to look at a book i saw last week. it's quite new and called
'How to Raise an Amazing Child the Montessori Way'
www.play.com/Books/Books/4-/1628463/How-to-Raise-an-Amazing-Child/Product.html
there are some excellent techniques that you can employ at home, from birth onwards, and i think i would prefer to use that to supplement DS1 & 2's experiences while keeping them in more 'standard' nursery environments.