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.
Sony's Firmware 3.51:
* Revisions to strengthen security have been added.
This is completely useless. Do not, and i mean DO NOT, use network update in your PSP. This is automatically update your PSP to 3.51 official firmware and you can't downgrade anymore. Thanks for reading.