I don't know when I first started to notice this problem, though I'm sure it's at least been a couple months. My recollection is that it wasn't always that way, so I went back through my old posts to find the last time I was able to post special characters and it appears to be March 3, 2012. I was surprized to see that I didn't make a single post between March and December of last year (that's got to be my longest dry spell ever). I think whatever is causing this happened sometime during that period.
By the way, the following are the characters I posted the last time it worked. Let's see if it works now.
? ?
(edit) Nope, obviously not.