I'm evaluating Team Foundation Server and am interested to know as much as possible about integrating it to external systems in particular the (custom) ones we use for source control and issue management.
Has anyone got any experience/insight/links to share on this topic?
There is a Rangers product on Codeplex to migrate or sync TFS Source Control with external systems. See http://tfsintegration.codeplex.com/ for more information on this topic.
Team foundation is centered around source control and provides most of what you would need around issue management (e.g. work items). If you already have both of those, I'm not really sure what you would be using TFS for.
I'll second what Ewald mentioned about the TFS Integration Platform-- that's a good start for converting from one source control system to TFS. Beyond that, there are a number of ways that you can integrate with the TFS Workitem store. There are other questions here on StackO that cover the details, but in a nutshell:
Excel Integration: TFS has excellent integration with Microsoft Excel, and this can be a way to do intermittent, one-way conversions of your WorkItems. In this approach, you export them to Excel, manipulate the results as necessary, then import them into your other system using whatever methods are available.
Programmatic integration: TFS gives you an excellent API for retrieving and manipulating WorkItems. It is a matter of a few lines of code to retrieve the WorkItem records that you want, then you can import them into your other system as appropriate.
A good example is how our Gemini Issue Tracker integrates with the TFS Source Control bit.
http://blogs.countersoft.com/index.php/2010/08/tfs-source-control
Not sure if this helps you -- it helps us as our "issue management" part is used by us internally as well as external partners, whilst the TFS source code bit is just internal.