I am currently working on this for our site, but I think it's a feature that may be useful to others.

It would be nice to have an indicator show on the order details in the admin to let us know if the customer has a client account, and if so, a link to hop right into their account where we can handle any changes there. If the client has private notes, they will show there on the order details page.
This will help us in many ways.
We can fix problems with clients saved addresses quickly.
We can see if the client has any special needs or history.
We can see if the client was not actually logged in when they placed the order and we can then link that order to their account.
Other stuff I am sure.

Good idea. Also, having a yay or nay client account indicator/link in the orders list would be helpful.