This issue is caused by the "script.aculo.us" v1.7.0 Ajax.Autocompleter IE bug. This appears to be fixed in the current version 1.8.1.
Next release of Click 1.5 M1 should upgrade to the latest library version 1.8.1.
The 1.7.0 fix for IE and that is to change the controls.js file line 87, "keypress" to "keyup"
Event.observe(this.element, "keyup", this.onKeyPress.bindAsEventListener(this));
This change has been checked into SVN and will be available in 1.4 Final
regards Malcolm Edgar