is the iPod Touch less able to connect to Airport WDS wifi network repeaters than an iPad? Kindle also doesn't want to! I have a wireless network in my apartment.  Because the apartment is large and walls are concrete, it was very hard to get wifi coverage throughout the whole place. Therefore I have one Airport Extreme, and then multiple Airport Expresses, and I have configured the airport expresses to join and extend the wireless network that is created on the main (airport extreme) unit.

The question:  When I am far from the original (airport extreme) unit, my iPad has no problem getting a four-bars connection from the satellite airport express that is relaying the connection. BUT, my ipod touch (and my Kindle also) can NOT connect to it. They just want to connect to the main central unit.

believe me I have re-configured this thing a DOZEN times... using the crystal clear normal method to get the distributed network going across all the airports... always the same problem. 

Has anyone found this to be the case -- that the iPod Touch, and a regular lowest-level kindle (bottom of the line unit as of 2012), will not connect to the WDS satellite, only the main unit?

[I will admit, i am only somewhat sure that the method I use, where I configure the units to "join and extend" the main network, is actually the same as WDS.]
I've pretty much solved this problem. I do not FULLY understand why, but now I can have my iPod touch (and I am hoping my kindle) attach correctly to the satellite/repeater airport express.  Here is what I did:

I did two configuration changes to my central hub airport extreme router (i.e., the one that my other routers were merely configured to "extend.")  I realize now that these are both configurations that must be done at that central router, and the repeater routers then simply mimic or reflect accordingly.

-- First, I used the most recent Airport Utility (v6.0) and clicked on my main router (which, crucially, is an airport extreme that has dual band 802.11, both 2.4ghz and 5ghz.) 

-- In the airport utility, I clicked on that Extreme router and then in its funky popup window, clicked Edit.

-- Clicked the tab called Wireless.

-- Clicked on Wireless Options button.

-- the first config change I made on that page was to change the Radio Mode setting to "802.11a/n - 802.11 b/g."
     What nobody really tells you is that that hyphenated choice means that the 5GHz band will use the first of those two (namely, a and n, in other words the strong modern N standard will be used on this band ONLY) ... while the 2.4GHZ band will use only the second of those two, b/g, in other words, the devices that only can use that older weaker standard.
     So, it's kind of a segregation. I'm telling it "No, don't let the 2.4 ghz band try to handle traffic using N. That band will just take care of the old crappy clients." 

-- the second config change that I made on that page was to check the check box to make a totally separate wireless network name for the 5GHZ band.  Further segregating things.  When I tried my solution without this additional step, my problem did not fix itself, though I don't fully see why.

As I said, a few questions remain in my head, but the darn thing works, so I'm happy. Among many other questions... are those airport expresses dual-band too? I didn't think they were but somehow they are repeating this dual-band strategy?  Whatever. it works.

Wanted to say thanks to Richard, whose reply caused me to get off my butt and try more investigating into this.