Sony made available today the latest firmware update for the PSP. Version 5.03 of the firmware is a minor bug-fix update with no specific feature improvements listed at Sony's official update page.
The update can be made through the usual channels. For more details, see the Sony update page.
This is the first update since version 5.02 hit on Novemebr 20, 2008. That update was also minor, fixing some issues with 1seg channel list display.