This is a new one for me.
Fresh Mango install, and the Modules -> Check for Upgrades says this instantly when I click the "check for upgrades" button.
ma.log doesn't record anything when this happens.
Checked out my DNS settings, and I can ping/nslookup etc from my command line, but Mango seems to have trouble checking for updates.
Not a big deal, since I can manually upgrade stuff, but I dunno why this is happening. First time I've seen this on a dozen Mango installs here now.