Using Rational Team Concert when disconnected from the server

I do not believe so.  To leverage RTC's features and functionality you need a persistent connection to the repository server and there is no way I am aware of to download all project related artifacts and then disconnect.

For source control, a user can create a repository workspace, load their local sandbox, then disconnect from the server and continue to make changes to the files in the local sandbox.  However, you would not have access to RTC source control commands.  Once you reconnect, the pending changes view should update and you will see unresolved changes for any repository workspace where you made changes in the sandbox while disconnected.

-Chris

If you are only interested in SCM data (change sets, components, streams), then yes, you can create a local RTC repository on your machine, and create your repository workspaces in that local repository.  You would then use "distributed delivery" to deliver changes from your local repository workspace to the central repository stream.  But all of your work item and planning information would be done in the central repository, and not while you were disconnected.