Just got back from Mazda dealer for this problem. There is a TSB that affects certain VIN 3's and 5's from 2006-2009. The TSB reads just like the write-up I gave to the dealer when I dropped it off last night.
"At highway speeds for extended periods of time during high ambient temperatures and high humidity, A/C may become erratic in operation. Heavy frost might appear on high pressure line and system may stop blowing cold air or working."
Saw this two weeks ago after 2 hours at 80 mph with the temps in the 80's. First the airflow was less, then warm and then not at all. Turned "Auto" function off which helped, but then just warm air came out.
There is a kit to fix this issue. The condenser gets too cold and freezes up at highway speeds, so the kit changes the airflow to the condenser, replaces the cabin filter with a different one and some sensor for the system so it doesn't get as cold. This is from the service manager at the Mazda dealer, who knows I do a lot of my own maintenance. I saw the TSB myself, but didn't get to read much of it.
$365 parts,labor and tax. Not happy about that, but each car has its issues and since it's out of warranty, I expect some hassles.
Update: I emailed Mazda about my problem and said though I'd like for Mazda to pay for the whole thing, I'd be willing to work with them. They responded quickly and have agreed to pay for parts and I pay for labor, cutting the bill in half for me.
That's way better service than I was expecting by all accounts. Wonder if I send them an email about putting the the Skyactiv diesel in the 5 what will happen