A friend of mine bought a Mini last week, serial code starting YM535 ... and it was one of the new ones. (I told him to check for YM537 before leaving the store, but did he? Hell, no.)
I would guess most of the older ones are out of inventory now.
I ordered mine on the 17th, still "preparing shipment". I'll let you know which one I get when it comes. Also, for those of you worried about getting the regular specs (not 1.5), you can enter your serial number on support.apple.com on the "About your Apple Product" before opening the box. From what I've heard, if it says "Late 2005" then you have the updated one! Try it out with YM5360V2TAC which I found on another site.