Change blindness is a phenomenon where humans fail to see major changes within the visual field regardless of how significant the change is. For a fun example of this effect, take a look at this. Or check out Wikipedia's page on the subject.
If you are wondering what this has to do with RIAs, lets think back to the web 1.0 days. In most applications the only way to see changes in data was to refresh the entire screen which resulted in a brief loss of focus followed by an attempt to regain focus once the page was refreshed. Usually the new page was different enough to make you briefly rescan the entire screen.
Now fast forward a few years and we are in Web 2.0 land with all the RIA goodness thanks to AJAX, Flex, Flash, etc. With these new technologies has come the ability to refresh only the data of the screen which, on one hand, is much better since you don't have to reacquaint yourself with the screen after every change. However, on the other hand, without page refreshes it may become difficult for users to actually notice that a particular portion of the screen has changed. For example, if I add an item to my shopping cart, I could miss the fact that my total or estimated delivery date has changed.
Keep in mind that if it is important that the user notices a change, then we must make it obvious (with a non-modal) notification.