Sony's PSP firmware update version 3.73 is available for download now in Japan.
A Web translation of Sony Japan's announcement tells us:
"PSP software-only play in the UMD loading screen to stop the symptoms has been confirmed for the UMD drive to improve operating stability."
So, improved stability for UMD. That's the only change that Sony Japan is telling us about. Meanwhile, Exophase reports
† that upon booting up the update, the following message appears in the changelog: "Support for PlayStation Network titles has been expanded". It's hardly pant-wetting stuff, we know.
Currently, other regions have not been offered a similar update.
Source: Exophase