It won't be fixed with a firmware update. As far as I know, the USB controller in the express isn't capable of sharing a USB HDD or thumb drive. Apple isn't intentionally crippling anything, it's just a feature of the Express. If you need that extra functionality, then consider upgrading to an Extreme or a Time Capsule.
Edit: If it was capable, I'm sure there would have been a hack released to enable the functionality, as was the case with the Extreme's Time Machine capabilities before Apple enabled it themselves.