I don't know why it wouldn't work if the url is correct. Have you ever used the Firebug addon for FireFox? It might let you see if a call is being made, or if there are any javascript errors occuring. Have you tried it without any onClick? (Probably you added that later as a test).
Also, the url you have above has two '?' in it, I think what you want is:
Code:
href="/showthread.php?t=$post[threadid]&do=link"
but I don't think that would keep it from working at all (it should still at least reload the page, I think).