I can help you a little there:
You are correct, 4.0 vSphere client cannot connect to a 4.1 host, unless it is a reconnect, for some reason. This however will only happen if you upgrade from 4.0 to 4.1 from the bundle, using vihostupdate for example. If the vSphere 4.0 client is connected to the host whilst it is 4.0, when the restart is performed and you allow the client to wait to reconnect, it will.
Also normally v4.1 client will only connect to a 4.1 host.
The thing to be aware of is that when you run the vSphere client application, all you ar really doing to running a launcher application. This has the window where you put in the server and login details. This will then start the appropriate viewer application.
This means that if you have the 4.0 client already installed and then run the 4.1 installer, you will end up with a client machine able to connect to either 4.0 or 4.1 hosts. The launcher will work it out for you.
One of my colleagues made the mistake of uninstalling the 4.0 client application before installing 4.1. A slightly annoying mistake as you are aware.
As an exercise, take a look in the VMware program directory. You will find a directly named "4.1" which contains the 4.1 viewer. In a dual installation there will also be a "4.0" directory.
The shortcut you click to open the client application actually point to a launcher application in what I think is named the "launcher" directory.