Is there any description/specification of the planned functionality of Design Manager ?

If you are referring to a high level description/specification for our 3.0 release, you can find that on our project pages. If you want more details, you can browse our online help.

http://publib.boulder.ibm.com/infocenter/rdmhelp/v3/index.jsp

How does it integrate with RSA ? Is there any support planned for WebSphere modelling tools (WS Business Developer, WS Integration Developer).

In our 3.0 release, RSA models are "imported" into the DM server via an application called the "RSA Import Engine" that is available on our download page. DM does not currently support WS Business Developer or WID but it does support RSA BPMN models.

Will the model artefacts be stored in the Jazz SCM repository and support full versioning, baselines, and snapshots ?

RSA models are stored in the Jazz (JTS) repository which supports versioning and baselines (aka DM snapshots)