Since yesterday evening, when I launch Steam on OSX, a self-update progress bar appears instead (at 0 of 30MB or so). This bar does not advance, an error dialog appears:

Steam needs to be online to update Please confirm your network connection and try again.

The app then exits. This happens whether wifi or ethernet or both are connected, and pings to the outside world succeed throughout. If I look at the logs in Console, they are very similar to this example (though that's not mine). Specifically:

Success! http://store.steampowered.com/public/client/steam_client_osx?date=718277
[...]
Failed! http://cdn.store.steampowered.com/public/client/breakpad_osx.zip.27f59114a86fcd50533e1d7b128f9300947f9969
Failed! http://cdn.store.steampowered.com/public/client/steam_osx.zip.11a99384214805f2dd3be5084ba6be61d662f8ac
Failed! http://cdn.store.steampowered.com/public/client/miles_osx.zip.d9fb546541f59c1fdd03962a605236b1021abab8

Requesting the first URL successfully returns some data including the filenames of the latter three, and requesting any of those gives me a 404 (I've tried multiple clients on multiple continents).

Searches on Google and Twitter show about 10-20 others having this problem in the past 24 hours, but hardly the angry mob I'd expect if the problem affected all Steam OSX users.

Things that have already been tried with no effect:

• Switching between wifi and ethernet.
• Killing all Steam processes including ipcserver.
• Moving the ~/Library/Application Support/Steam/registry.vdf file away.
• Requesting those URLs with other clients and from other locations.

Interesting: that first URL with the date parameter returns the same content even without that parameter (thus would lead to the same 404s) suggesting that the problem is not necessarily specific to coming from a particular currently-installed version of Steam.