here are the answers to your questions:
1. Current versions of Sitefinity are built against .NET 2.0, but you can change it to .NET 3.5 in your project. From the next version, Sitefinity 3.6, we will move to .NET 3.5 as the default build. 3.6 is scheduled for January of 2009.
2. Yes, Sitefinity works with SQL Server 2008.
3. Sitefinity works great with controls, since every page in Sitefinity is composed of controls. You can easily add new controls to the Sitefinity toolbox, regardless if they are User Controls or Custom Controls (compiled into a .dll file). The really nice thing about these controls is that you don't have to customize them to Sitefinity in any way. If they work on a normal ASP.NET page they will work in Sitefinity pages as well. As for third party controls, yes, you will be able to use them. Take a look how we added to the toolbox some Telerik's ASP.NET for Ajax controls or some standard ASP.NET controls (e.g. XmlDataSource).
All in all, Sitefinity is quite a good choice when it comes to migration. Due to it's fully exposed API you can write rather simple transfer functions that will transfer the data - API can also be used to create pages dynamically, add controls to them, change the themes and so on.
I hope you will find this information useful. Let us know if there is anything else we can do for you.
the Telerik team
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