Yes, I got the letter also. Here's an excerpt:

"Product Update: IMA Battery Software Update

Dear Accord Hybrid owner:

This notice has been sent to inform you of a potential problem with your vehicle and what you should do to resolve it.

What is the problem?
The computer software in your vehicle needs to be updated. Without the update, a technician using a scan tool in generic mode on your vehicle could cause damage to your vehicle's electric motor battery and/or cause the engine computer to falsely signal engine misfires. Additionally, we also want to update the original PGM-FI software to the most current version number.

What should you do?
Call any authorized Honda dealer, and make an appointment to have your vehicle updated. The dealer will update your vehicle's computer software. This work will be done free of charge. Please plan to leave your vehicle at the dealer for half a day to allow them flexibility in scheduling."