Workflow and Design Feedback on an Add / Edit Form
This is my feedback, Please let me know if I am misinterpreting :

1) Storing the entire PersonEntity object in viewstate will have following problems


It will make viewstate heavy and thus make page load slower.
It may not represent the current state of the object. 


You can store the personId, recreate personentity object and then call save. 
A lot more about viewstate here http://www.freshegg.com/blog/creating-lean-fast-web-pages-view-state_3377

2) You can make the LoadById method static