That was very quick. After the Sony PSP firmware downgrade hack has been available only a couple of days, Sony released a new firmware 2.01 to fix this 'security' issue.

Sony released the PSP firmware 2.01 only in Japan so far. Sony warns specifically that the update is only for Japanese PSP-1000, PSP-1000K and PSP-1000KCW.
The firmware 2.01 download is 16MB. There are no new features and bug fixes mentioned. Sony asks, as far as I read the translation right, for users cooperation to update.
More details in this Sony Firmware 2.01 announcement (Japanese).