While playing with the beta client I took a look at the packets it was sending and noticed it connects to api.steampowered.com. I've been trying to get something out of it and so far have found two types of request:
This appears to be the handshake the client performs at startup. The post data looks like:
Format is the desired response format, so far I've found that vdf, xml, and json are valid. The last 3 are self-explanatory. If the credentials are right the response contains a token that can probably be used for all sorts of cool stuff that requires an account.
The other one looks like:
format is the same as above, appid is the game's id in the store, count is how many news items you want, and maxlength is the maximum length of the item's content.
I think I speak for every developer when I say that I hope this is finally a real API we can use, and hopefully get Valve to document it. Has anyone else noticed this /or found some more requests?