It is not a driver problem, but a problem in the BIOS emulated by bootcamp on some (not all) MBP models. So unless Apple fixes it in bootcamp, there is no real solution. And obviously, Apple doesn't care. They are dropping the Expresscard slot anyway.
Unfortunately, this poster seems right: http://forums.macrumors.com/showthread.php?t=912259 :
The 2008/2009 Macbook Pros don't allow to use the ExpressCard slot because of EFI/Bootcamp errors (Windows Vista/7 cannot allocate an IRQ to the slot). You don't need any drivers for using an ExpressCard slot on Windows, you only need the drivers for the corresponding card you are using.
Bootcamped XP allowed to use the slot, but you could not use hot-plugging.
And from the Sonnet site ( http://www.sonnettech.com/product/tempo_sata_express34.html ) :
Tempo SATA ExpressCard/34 not supported under Windows 7 or Windows Vista when running on unibody MacBook Pro computers. The problem is caused by the MacBook Pro's BIOS emulation, which doesn't allocate enough resources to support certain kinds of Expresscard/34 devices, such as a SATA controller, in these operating systems.
(added link to Sonnet's explanation)