For those who may want to see the items still regardless the length. Or like in my case, doing cross application posts and wanting the text parameter to be passed with comma separated string.
I used the following work around but it isn't perfect:
On your RadComboBox, set the OnClientDropDownClosed and OnClientLoad equal to prepField and then drop the following JavaScript on your page.
This does not help w/AllowCustomText set to TRUE, however you could probably do something with OnClientTextChange but make sure you don't get stuck in an infinite loop. We have temporarily disabled this since none of our users complained about the certain circumstances mentioned above by Simon that it didn't work anyhow. Hope this helps.
Shane