Sony released Firmware 3.73 for the PlayStation Portable, earlier today. The firmware update is a minor one and simply correct a bug in the system software. The update is only live on the Japanese PlayStation website, but the update can be downloaded directly to the PSP through the Network Update feature. According to a rough Japenese-to-English translation, the "PSP software-only play in the UMD loading screen to stop the symptoms has been confirmed for the UMD drive to improve operating stability."
UPDATE
The US changelog is now available. The update improves the stability of the UMD drive when loading data:
Operation of the UMD drive has been improved to address cases in which
the UMD drive occasionally suspends and fails to reload data. Updating
the system software to version 3.73 will resolve this issue.