Is RESTful WCF service a fully released version or its a BETA version, because when downloading the msi is 'WCF_REST_Starter_Kit_Preview_2.msi'. What does this Preview_2 means?
The WCF REST Starter Kit Preview 2, is as the name suggests, just a preview and not a proper release (RTM/RTW). You may find that this related question helps answer your question:
REST capabilities are included and supported in the WCF that is part of the current .NET Framework, v3.5.
The REST Starter Kit is a bunch of add-on stuff to WCF for .NET 3.5. The starter kit is at preview 2. The starter kit is not necessary to do REST with WCF.
The WCF REST Starter Kit is intended as a vehicle for the team to get feedback from the community on new feature ideas around REST in WCF. In that sense the starter kit will probably always maintain its "preview" status and never ship as a standalone production release.
Having said that we are actively working on incorporating developers' favorite starter kit features in the .Net Framework - for example you can use the help page feature in the recently released .Net 4 Beta 1 release. As we get feedback on starter kit features we consider them for inclusion in the framework, which you can use in production.