Silvery - 9:00 pm on Feb 2, 2010 (gmt 0)
Yes, I'd actually written an article describing these near the end of last year -- I refer to them as "Landmark Icons".
As you noted, more popular business types tend to show up at the higher zoom levels, such as restaurants, hotels and entertainment venues.
I believe that attaining landmark icon status means that Google rather trusts your business listing, and that you've got slightly higher popularity.
However, I can point to lots of instances where relatively boring/unpopular businesses also are appearing, including lawyers and accountants.
What's really interesting is to zoom into dense shopping centers like malls and office buildings, to see which businesses in them appear as icons on the map.
Many/most are linked to their Place pages when I've explored them.