Had one of those 'doh' moments and checked the Apple store web site. They have a Kensington LiquidAUX unit listed there:
http://store.apple.com/us/product/TR...r&mco=NzYwMjY2
$79. Ouch. But it does come with a wireless remote. (Cool, but useless to me.)
And a more expensive deluxe version with a mount/dock, for $99.
Not listed specifically as 3G compatible, but several reviewers are claiming it works fine with no error messages.