Hopefully, by now folks have seen the official announcement about the release of the WCF Data Services October 2011 CTP. This release includes a bunch of new "in progress" OData v3 features, such as actions, spatial data types and operations, and vocabularies. The exact extent of the support in this CTP is detailed in the post Announcing WCF Data Services Oct 2011 CTP for .NET 4 and Silverlight 4.
In addition, this release contains the OData Library, which is designed for people who only want to generate OData-compliant messages and will handle everything else themselves without even using WCF Data Services (this is a great option for products that are already offering REST services but want to add a low-cost OData head).
I should also point out that this release also includes an out-of-band update to add OData v3 support to Silverlight 4, which is basically the same OData v3 functionality of the full client, but with only asynchronous APIs. However, there is a big new fix in the Silverlight library that enables you to access the underlying HttpWebRequest from the Request property of the SendingRequestEventArgs class. This is a big deal for people trying to set HTTP headers not explicitly exposed by the client, and for users who need access to HttpWebRequest goodies, like the CookieContainer.
We also have the documentation for this CTP published (for the first time) to the MSDN Library. This documentation is a bit on the light side, in part because most of the new OData v3 functionality featured in this CTP are not supported for the Entity Framework provider. OData v3 features added in the previous CTPs (like named streams) are included in the documentation.