Edit in response to early answer and comments
The biggest tangible argument for support .Net 4.0 apps is developer productivity - due to language enhancements, but also due to access to a greater amount of pre/built functionality.
I am trying to understand the risk and/or cost to rolling out software that was created and tested by the same company that wrote the operating system. I can certainly agree that it is not wise to make changes to deployed systems if there were no benefit. There are certainly benefits, and I am operating under the (possibly false I suppose) assumption that there is neither a lot of cost OR risk in such a deployment.
Does anybody have an answer about the risk and cost?
I am a developer at a fairly large company - the user desktops are all currently Windows XP SP3.
I am trying to make a case for approving the .Net 4 framework for installation on user desktops to support new LOB applications written in .Net 4. The approval process, I am told, is to install the new software and then test each and every other piece of software that is already approved to make sure something doesn't break.
Aside from this hurdle, are there any other arguments I should expect from Admins/Desktop support?