I have been using Ranorex to test on IE7 and 8 and have had no problem selecting items from combo boxes. However I am now using the same code to look at IE6 and it won't work:
selectItem(SelectTag selectName, string itemToSelect)
{
selectName.Focus();
selectName.Click();
ListItem listItem = "/container[@caption='selectbox']/listitem[@text='" + itemToSelect + "']";
listItem.Select();
listItem.Click();
}