There appears to be an issue that has always been present but it is now coming to light with the new user interface. If you are using the new ui, have a large picklist, perform a rerender, and are using Firefox 3 the panel that is being rerendered flashes blue. I did some quick testing with the old UI and this behavior is there but the panel flashes white and is barely noticeable. The new bright blue background is obnoxiously apparent with the new ui.

What is quite bizarre is that if the picklist has a selected value that is less than 20 you will not see the behavior. Perhaps this is Firefox's fault but I would think there is someway to prevent the bright blue flash by updated the CSS or javascript that performs the rerender. Below is the code to reproduce and there is also a video that shows how annoying this can be from a user experience perspective.

<apex:page controller="rerenderFlash" >
	<apex:form >
	    <apex:pageBlock id="block">
	        
	        <apex:pageBlockButtons >
	            <apex:commandButton value="Rerender" rerender="block" />
	        </apex:pageBlockButtons>
	
	        <apex:pageBlockSection >
                <apex:selectList value="{!selectedValue}" size="1" id="number" >
                    <apex:selectOptions value="{!options}" />
                </apex:selectList> 
	        </apex:pageBlockSection>
			
	    </apex:pageBlock> 
	</apex:form> 
</apex:page>


public class rerenderFlash {
   
    public List<SelectOption> options {get; set;}
    public string selectedValue {get; set;}
    
    public rerenderFlash(){
    	options = new List<SelectOption>();
        for(Integer i = 0; i < 100; i++){
        	options.add(new selectOption(String.valueOf(i),String.valueOf(i)));
        }
        selectedValue = '90';
    }

Video: http://vimeo.com/10013511