<p>My company has developed (and still continues to develope) a large ASP.NET business application. Our platform is ASP.NET 2.0 using some ASP.NET Ajax. We're <strong>extensively using third-party components</strong>, like webgrids, comboboxes, treeviews, calendar and scheduling controls etc.</p>

<p>Now, I don't know a lot of ASP.NET MVC and I'd like to know if there <strong>is a way to use these third-party-controls in the ASP.NET MVC model</strong>. Or do vendors have to rewrite their products in order to make them suitable for ASP.NET MVC?</p>

If they use the ASP.NET Control Model (That will be about 99,9% of controls written by ASP.NET Control vendors), they have to rewrite their controls. How much work there is in that, is very different depending of there arhitecture of their controls - the more ajax they already use, the more posible it is that they easily can change it to MVC.
ASP.NET AJAX Control toolkit for exsample can work with MVC. You can see how to do this in a video on WWW.ASP.NET:
http://www.asp.net/learn/mvc-videos/video-373.aspx