As luck would have it, I found a workaround shortly after posting this. If you add the 'prompt' property (e.g. prompt="Select") to the combo box such that it does not display blank initially, it fixes the problem. Further evidence I think that this is a bug since I can't think of a logical reason this should have fixed it.
i have a flex 3 project someone developed it and he wirte the code such as the below example:
i have similar problem as you mentioned above, but it seems he use a different approach to create the ComboBox in the mxml file, and i added the prompt property such as you mentioned befire, but unfortunately it still not working, do you have any idea !!!...
thanks in advance,
Try using the valueComit property on mx combobox.