Just downloaded the shopping assistant for Mac. Apparently, its now available - through I signed up to get an email when this happened, I didn't, but found it randomly. Showing Mac OS X as the requirement.

Download was pretty simple. Though I like the old one much better. The mileage earned no longer shows up on the assistant itself. Instead, when navigating to a participating site, there is a pop over that comes down from the top of the window listing how many miles you can earn, then you have to click a button that says "earn miles." Once you've done that, it redirects you to a new version of the homepage, and produces a new pop over that says completing a purchase will earn X miles.

As previously noted, no counter for searches any longer. But UA logo pops up beside items on search items found, whether done from the toolbar, or from a non-toolbar search on participating search engines, to indicate the site participate in MP shopping.