I'll add a bit of feedback but my issue is still not sorted.
When trying to reproduce the problem in a new (empty) project, I noticed a couple of things:
1. When defining the RadComboBox directly in my aspx page, it worked immediately.
2. When creating the RadComboBox via code, it didn't work. My colleague then suggested that instead of adding the control to the page i.e. Page.Control.Add(combo), that I add it instead to a Panel. This solved the problem in test website but it still not displaying any of the requested data for some reason, even though the source is binded in the ItemsRequested and my code is identical as the one used in point 1. which displays everything correctly.
I thought this could be the problem in my main app, but my RadComboBox is not added to the page directly. It is added to a TableCell which belongs to RadGrid but to be sure, I added the combo to a panel and then added the panel to my TableCell but it made no difference! It still doesn't trigger the ItemsRequested event.
The Telerik version I'm using is: 2010.2.713.35, if that's of any use.
Could it be a bug?
it behaved exactly as my original question did i.e. ItemsRequested is not triggered when clicking on the drop down button.