Here is current situation with SDIO support.
I talk with owner of Zaurus SL-C860, and he said:
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I do have older Zaurus SL-C860. The WiFi SD card from
http://www.c-guys.net/ works only with C860 and C760 original japanese
roms, not with SL-C3100. The problem is that they never updated their
drivers.
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Also, I did wrote to EmbWise about their SDIOWorx technology, and they said:
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We have a working SDIO-WiFi drivers on the platforms that we support our SDIOWorx with.We have ported our SDIOWorx onto ONLY Zaurus SL-C3000 and not all of Zaurus PDAs.However we would think that the stack and the driver would work on Zaurus SL-C1000 as well as SL-C3100,since they have the same hardware architecture.But we don't want to commit till we decide to and test the software on these other models.
The WiFi driver is being tested now on SL-C3000.
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I suppose that C-Guys will not support new Zaurus' in the future, but coming soon SDIOWorx will hopefully solve problem with SDIO for all Linux-based PDAs which can support it, including new Zaurus SL-C3100.