I was browsing the support for ExpressCard in Linux. I couldn't find any explicit support for it at all, not matter where I looked. I found usb support and pci express support, as well as hot plug versions of the latter. This puzzled me a bit, since I'd read that Linux supported ExpressCard cards. Maybe no special software support is required at all...
So, I thought I'd get a couple of ExpressCards and take them for a test spin. It turns out that FreeBSD versions back to 4.x that can boot on modern machines with ExpressCard slots will support one important class of cards (assuming the drivers are in the version of FreeBSD that you try).
The ExpressCard standard specifies two types of cards, from an electrical point of view (not to be confused with the two width form-factors offered: 34mm and 54mm). The first kind is card that contains a USB 2.0 device in it. In this configuration, the host bridge connects it up to power and a standard usb hub. The second kind of card is a single channel PCI Express card, used for applications where greater bandwidth is required between the CPU and the hardware on the card.
The USB versions just work. I plugged in a Delkin Devices eFilm ExpressCard that I was recently able to purchase due to the generosity of a someone who sent me money to buy toys with. This device supports SD, SDHC, MMC, MS, MSpro and xD cards. Since I also wanted to enhance the SD/MMC stack to support SDHC cards, I thought this would be a good choice (since the price was right) for me to test things out on.
The PCIe versions will need PCIe hot-plug support added to pcib (or a subclass). I've had trouble finding cards that are PCIe for sure, so if you know of one, please let me know. I'm guessing that the eSATA cards are a good choice for that, but confirmation would be great.
So the good news is: FreeBSD has supported at least some of the ExpressCard cards since before the ExpressCard standard was published.