FWIW, I'm completely with the other poster. I just ripped out an old control from one of your competitors and replaced it with yours in our application. The hair-pulling I had to go through and the extra code I had to write to bypass this issue was totally unnecessary. If you have an internal issue in the design of your control, like this one, I really believe you should do one of the following:
1) Abstract it out so your customers don't have to see it. If someone tries to set SelectedValue or SelectedIndex, store the value internally, wait for the databind, and set it. Do whatever is necessary so your control acts in a way that users would expect after using the System.Windows.Forms.ComboBox.
2) If you just don't have the time and resources to do the former (and can't fix the issue altogether), then at least throw an exception if a user tries to set SelectedValue or SelectedIndex before the datasource has been bound. In other words, make it easy for the developer to figure out what they did wrong, rather than having to pull hair and then spend time Googling for this forum post (or, worse, waiting to hear back from Telerik while under a deadline).
It's issues like this that cause me to have conversations with people, at least once per month, that go like this:
Person A: Who's controls do you like best for developing your app?
Me: Well, Telerik is hands-down the best, but not for the reasons you think...
Person A: oh?
Me: I've used other controls, and while Telerik's got everyone beat on the number of features and controls, but the number of times I beat my head against the wall with silly things is frustrating. They've got great support and helpful engineers... but sometimes I wonder if the cost/benefit ratio is as good as I think it is. When I consider all of my time spent researching stuff I don't have to with, say, the built-in .NET controls, I'm just not sure it isn't costing me more wasted time than I think. I recommend the Telerik controls pretty strongly, but make sure to increase your budgeted time for the "unknown trivia" you'll discover with their controls.
I really love Telerik's products, but I really feel like you need something or someone at your company to help you see how unintuitive your controls can be sometimes. Maybe it's a person with the title "UX Director" (UX = User Experience) who focuses solely on intuitive interfaces and APIs (i.e. the developer's experience). I really don't know, but some days, like today, I see 4 hours wasted getting something as simple as a ComboBox working, and it really makes me wonder...
Patrick (happy customer but unabashedly tough critic)