If you need to disable the link, you can do so in other ways. I don't think it's unreasonable to have a link positioned somewhere tactful in the footer.

If your clients don't want to see the link, make it the same colour as the background. That way search engines will still pick it up.

Personally, I disabled the default one and made my own link that states just "Powered by Ubercart"

The more I use Ubercart, the more I see that it's the best open-source cart on the net today. It's just so extensible and configurable - and it's only getting better. Thanks guys for all the hard work. It's really appreciated.