What is the adoption rate of .net 4.0?
When will it be on Windows Update?
I heard that it is smaller than 3.5, does that mean if I install .net 4.0, it will decrease the space used by the framework?
If I make an application in 3.5 and it runs on a computer that has 4.0, will my application get some of the optimizations that 4.0 uses?
You'll find most people probably won't install it until they have to use something that needs it.
It's an optional update on Windows update as far as I know. Optional updates tend to be ignored by a lot of people.
The install is small, but it won't make your .NET install any smaller. It will increase.
No. It will run on the .NET 3.5 platform. To use .net4 features etc you have to convert it to .net4.
In terms of size, the Net Framework 4.0 Client Profile is what your users will need to install to run 4.0 applications and it's a 41MB download ( http://www.microsoft.com/downloads/details.aspx?familyid=E5AD0459-CBCC-4B4F-97B6-FB17111CF544&displaylang=en ) so still not exactly 'small'. It's much better than the 3.5 client profile though, which varied in size depending on what you presently had installed but could be 100MB+
If people are installing over the Internet the web installer, http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=5765d7a8-7722-4888-a970-ac39b33fd8ab, which is 868 KB can be a better experience. This is a small installer that gets up and running quickly. Then during the installation it downloads only what you need for the machine it is running on. For example, on x86 the downloadd is around 28 MBs.
In addition, in the standalone instructions there are links for x86 and ia64 standalone installers. Although unless you can guarantee you will only support x86 machines for example you should use the web or x64 installer standalone installer. You cannot install x86 only on x64 machines.