Got this response from Garmin to my question.
I can't say for sure, but I am assuming that Google is receiving updated
NavTeq data once per quarter for their website. They don't have quite the
conversion work to be done, nor the time spent publishing CD's. We receive
an updated data set once per year from NavTeq; from which we publish an
updated version of the City Navigator CD.
The version 6 that is being used on your 2610 was released in August of
2004. We hope to have the next version out sometime early this summer.
This should show the Sun City Hilton Head area that is missing right now.
As an existing customer, you will be notified via email when a new version
is released. We appreciate your patience and apologize for any
inconvenience this has caused you or your wife.
Thank you for your interest in Garmin GPS. If you have any further
questions, please don't hesitate to ask. Please include all previous
correspondence when replying to this message.
Best Regards,
Matt