According to posts on RimfireCentral.com, they're selling for considerably less than MSRP. (A price of $389, in at least one instance, was quoted for the blued Target version.) And they seem to be cropping up at local gun stores and gun shows. Ruger appears to have filled the distribution channels before making the formal announcement.
Personally, I don't care. Having all the other Marks in the series, I'm prepared to pay any (reasonable) price to get one of these.
Meanwhile, I've been studying the exploded parts drawings. The general layout of the internals is the same as in the previous Marks. The main differences (aside from the simplified takedown) are a new u-shaped magazine disconnector that is inletted on both sides of the hammer, and a redesigned sear. Therefore, it would be very simple to remove the magazine disconnector, provided you substituted either a Mark II hammer, or a Mark III hammer with a Tandemkross bushing. (I think replacing the sear would be a good idea also.) In my opinion, it's just a matter of time before Tandemkross makes a double bushing, so you could use the Mark IV hammer without the magazine disconnect.