This feature has been implemented in KPI Dashboard in Q2-2011.

At Twitter and Yammer you can follow people. At KPI Dashboard you can follow.... KPIs. Would it not be cool if you could subscribe yourself to a set of KPIs and you would receive a daily or weekly email with the progress of those KPIs during that week and have the ability to comment on them via messages?

Let us know if you would like us to implement this feature.