I covered the internal details of the SD Wi-Fi card's design in the original review, so won't waste space with them here. The new card will be readily identifiable by new packaging which has a checkbox for the Zire 71 right on the front (Figure 1). By the way, the new card's drivers also support devices running Windows Mobile 2003.
Figure 1: The new package
(click on the image for a larger view)
The new SD Wi-Fi is still based on SyChip's WLAN SD6060 reference design, but has hardware tweaks that SanDisk says make it different enough from the original SD Wi-Fi card that the original cards can't be used with the new card's drivers.
Being the curious kinda guy that I am, I popped the original hardware version 1.0 card (this is marked on the sticker on the back of the card - the new card SanDisk sent me was marked H/W: 2.1) into the Zire 71 and ran the same FTP test that I used to test the new card (more later). To my surprise, the card came right up, worked with the new utilities and had essentially the same performance as the newer card.
When I reported this to SanDisk, they said they were aware that the older card would work, but that it will connect to only about half the APs that the new card will. So if SanDisk makes the new drivers available for download, I guess you'll be able to give the old card a try for yourself.