I have seen a few queries on the Technet forums recently regarding the use of ASP.NET 4 and SharePoint, an example being this post here.
There appears to be some confusion (myself included) around whether or not ASP.NET 4 is supported for both MOSS and SharePoint 2010 - in short, the answer is that it is not currently supported in either product.
Whilst it is possible to use ASP.NET 3.5 in MOSS by making some modifications to the SharePoint site web.config (instructions for doing that in MOSS here; SP2010 uses 3.5 by default), this is unfortunately not the case with version 4.
The reason? ASP.NET 4 uses a new version of the Common Language Runtime (CLR). Whilst .NET 2.0, 3.0 and 3.5 all used version 2.0 of the CLR, .NET 4 uses version 4. I'm not entirely sure what happened to version 3. There is a decent post on stack overflow with further details.
For those that are wondering, the fact that .NET 3.0 and 3.5 do not show up as options within IIS is also related to the CLR version. IIS knows about that - it does not know about framework and compiler versions and the aspnet_isapi.dll from the version 2.0 is used for version 2.0 through 3.5 for this reason. This post explains that in more detail. Does that mean we will get a new option in IIS for .NET 4? You bet, due to the use of CLR v4.
So does that mean we are destined to use .NET 3.5 for the lifetime of SharePoint 2010? Not necessarily. Aside from the possibility of Microsoft adding support for version 4 in a future service pack, those using ASP.NET Web forms in separate virtual directories will still be able to use .NET 4 so long as said virtual directories are in a separate application pool. I have confirmed this is the case with Microsoft in this post, and believe this to be the case for SharePoint 2010.
I hope that helps clarify things a little.
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