According to an article released today, after you install Exchange 2003 SP1 you may experience a crash in your Information Store because of malformed email messages. Should this prevent you from installing SP1? No. But you will want to install this fix (after testing this in your test environment) to prevent crashes. The official Microsoft response is in the article: "Only apply it to systems that are experiencing this specific problem." But, in an effort to be "Honest and Respectful", no one wants to experience a crash (especially the Information Store), right? Let me say one more time that you should thoroughly test this hotfix before installing this in your production environment.
This hotfix is not available for download without calling Product Support Services (PSS) for the hotfix and does require a restart of the Information Store.
872963 Information Store crashes periodically after you install Exchange Server 2003 SP1