In a follow up to a previous question, would you avoid using a component library that required .NET 3.0/3.5? Would the additional install requirements be too much to ask or, is the .NET installer easy enough to just include or link to that it doesn't matter?
It would totally depend on what I was writing. If I was writing an application that targeted a large consumer base, I would probably stay away from something that required 3.5 because of the size of the download. If I was writing a Visual Studio 2008 add-in, I'd have no qualms about using something that required 3.5. If I was writing an internal app, it would depend on the current desktop configuration and how open the IT admin folks were to rolling out 3.5 if it wasn't there already. You will probably need to as about a more concrete scenario to get a more concrete answer I'm afraid.
I might avoid the component under several circumstances. The ease of use of the .NET runtime upgrades is not the only factor. If your application is going to be used in a corporate setting or you do not have a technical audience I would take a lowest common denominator attitude. If your audience is less constrained you may have some justification to lean the other direction. Another important factor are the features the component brings to your application. If the feature is significant enough or important enough weigh that against the users time in configuring their environment. The environment will likely catch up but these other external factors may be important.
To me it is more a question of when rather then if. Lets face it, virtually everyone will be upgraded to .NET 3.5 sometime, if not already.
Quite frankly, there is no good reason I can think of why a Windows computer should not be upgraded to .NET 3.5.
That having been said, if I had a client that for some reason refused to accept any software that required .NET 3, then I would have to accomodate them if I expected to get paid.
As for the installer, it really is no bigger deal to install the latest .NET then it is to install .NET 2.
There is no real need to avoid. There are lot of tools/installers out there that can make the installation seam less and ubiquitous. Remember you need .NET 3.5 SP1.
Note: Your clients need to have administrator privileges to do install .Net if it is not already present. This can sometimes pose a problem in terms of angry customers. Remember the article on MLB dissing Silverlight primarily because the users could not install the plug-in as they did not have sufficient privileges at work.
Lack of support for Windows 2000 is sometimes a reason to avoid .NET 3.5.