Although we haven't yet seen malicious attacks via the JailbreakMe vulnerability, we recommend to install the patch right away.
This does mean that users who have jailbroken their devices and prefer to keep it that way will have to face the increased likelihood of malicious attacks through this vulnerability.
We recommend that all iOS users, including those who have jailbroken their devices, would install the latest update now.
More details on what was patched is available from Apple.
Updated to add: Jay Freeman (Saurik) has made an unofficial patch for one (CVE-2010-1797) of the two vulnerabilities patched by Apple. It's available for Jailbroken devices via Cydia, and will work also on the older devices that have not yet received any updates from Apple.