It's only been a couple days since the Lumines exploit was released, allowing all PSP firmware versions to be downgraded. And now, Sony has released Firmware 3.51 to combat this exploit. No new functionality can be found in 3.51, it only patches the Lumines exploit.
Changelog is as follows:
* Revisions to strengthen security have been added.
If you do wish to update, grab the new firmware either through Network Update or via the download link below.