Now that mega gig SD cards are in sight, one, large internal card may be all that is needed by many users. This begs the question about alternative uses for the external card slot.

Could a future WiMAX, TV tuner, USB, Ethernet, toaster oven, etc., SDIO card be used?

Cards are only recognized with the door closed, so: no.

Well, fortunately this is not limit in hardware but pure software choice in linux kernel. Detecting cards when inserted without closing doors is change in one line in omap mmc driver source. I have this change on my N770 to read full sized MMC cards. Works on N800 too but has no real use.

As for SDIO - there is currently no support for generic SDIO in linux kernel nor for any specific SDIO hardware.

SDIO is supposed to be coming soon, isn't it?
http://linuxdevices.com/news/NS6177996610.html

I hear ya...

The switch Karl speaks of can be turned off and the properties of any card can be read... There just isn't any compelling reason to do so, IYHO at this time.

That info is good to know in this world where anything can happen and usually does.

First of all, is SDIO supported in hardware or not? Someone will need to figure this out.. it's completely possible to have an SD slot that only supports memory cards, AFAIK.
Secondly, I believe I have seen that picture in the OP (or something like it) before: When SDIO was introduced there was a lot of ideas about what it could be used for, and I saw a picture like that on some of the web pages. Old pages, years old.. The problem is, most on that list of possible future cards never came to anything.. with the exception of wi-fi SDIO, where there are a few cards (< 10 I believe) on the market. There's a bar code reader out there, and a few BT cards. I've seen some announcements over the years for some specialised cards, and a TV tuner, but nothing in the shops so far. Most of it simply didn't happen.