I implemented a shared calendar view for
CRM4 at a client, it shows appointments across many different business units and account in consolidated views, we used a third party share point web part by Bamboo Solutions, unfortunately there were many limitations to the control that meant that the implementation was lacking in features, such as adding a new appointment from the control!!
I've come across an excellent calendar control leaving me in doubt that if we ever have to implement a calendar for
CRM again this is the control we will use.
Feature rich, excellent usability for the user and relatively cheap. There's a demo page
here 
