I've searched all over the internet and have yet to find a good Lotus Domino or Lotus Notes Visio Stencil for use in diagrams and slidedecks. I created this for personal use and am sharing it for the same. It is not perfect and won't be supported but I think you will find it is pretty useful. It will work with Microsoft Visio 2007 & 2010, just copy it to your My Documents\My Stencil folder for it to be displayed in your Visio program.
Link to download:
Lotus Domino Notes Visio Stencil VSS