You can use ajax to accomplish that. You can use a library such as ajax4j (part of richfaces) to have ajax support for JSF 1.2 - if you use JSF 2.0 (very new, so not very likely) you'll have ajax support by default.
myPanelId refers to an ajax4j region that should be rerendered.
See documentation on ajax4j website, and the many examples on the web for more information.
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