Still not able to provide the 10watts of power from the ExpressCard slot; would require an external power supply.
And as both you and I state in different ways, will also not meet the performance requirements.
I assume the original question was about the ExpressCard slot on older, existing laptops. Not some new enhancement of an ExpressCard implementation.
Bottom line, it's not going to happen - there won't be any Thunderbolt card for current ExpressCard slots.
Thunderbolt is the new expansion capability for laptops, and some desktops and all-in-one computers.