Steam appears to be having some issues (Updated)

Steam's concurrent users took quite the plunge at around 12:45 PM PDT.

Update: Whatever happened, it now appears to have cleared up. I'm currently able to log into Steam without delay, start games without sync errors, and list items on the Steam Marketplace, and Steamstat.us is also showing green almost across the board. Dota 2 services are still coughing up an HTTP 503 error and player inventories are "delayed," but otherwise everything appears to be back to normal. We'll update again if the situation changes, or if we hear anything from Valve.

Original story:

Are you having problems accessing Steam? You're not alone. We're not sure why, but something, somewhere, appears to have gone very wrong. 

The problems appears extensive, based on Steamstat.us. The store is up, but virtually everything else is down, "unknown," or struggling with heavy loads. That means cloud saves, file syncs, and even logging into the service either aren't available, or will take much longer than usual. Anecdotally, we were able to access the Steam client about five minutes after launching it, today at about 1 PM PDT.

When we attempted to list an item on the Steam Market, we got this message: "We were unable to contact the game's item server. The game's item server may be down or Steam may be experiencing temporary connectivity issues. Your listing has not been created. Refresh the page and try again."

I've emailed Valve for more information, and will update if and when I receive a reply—or when the service is back online.