I have wondered too that what letters and characters really is "official" that could be used in SSID, but I can't find any (at least) moderately definitive answer from Internet...
I do know that some times example space works in SSID, also finnish letters like Ö and Ä, but with some card/driver combination one or all of those characters in SSID makes it unconnectable from the computer in question...
So best answer I can give is that to be on most safe side do not use any special characters in SSID, but if it is enough that it works woth your stuff, then just test it
i know it's a long time ago i created this, but thought i'd respond any ways.
Sash
Yes, ex. My street name is Hørgårdsvej 61, if i try to write that into dd-wrt it'll say illegal ascii code.
Notice that standard firmware on my netgear and linksys didn't do that, and have experience with apple airport and airport extreme and they don't either.
But i have seen situations where the ssid has been broadcasted as H[rg{dsvej, where ø and å has been replaced by some other characters.
Running v24-sp2 build 13064
micro on linksys wrt54gv5 and mini on netgear wgr614l