I too am experiencing this issue - but with a twist. After reading the solution provided in this forum (Thanks mulawingalang!), I reset my A500, and it worked the first time. But, as I read further on, I saw the post by jkscb4, and thought a second forget/reset wouldn't hurt, just in case it was the magic number (along with setting my jaw in an heroic grimace). I continued working, but 24 hours later - no Wifi even after 4 resets. Then this morning I try another reset - and it works! So at best the solution is transient, at least for me, and stability is a big issue for the 3.1 update from Acer.
I would like to point out that we shouldn't be too quick to blame Acer 100% - maybe 98%. Who among us didn't complain to Acer about the delay in getting the update to us, when every other device had received it - I know I was po'd, and let them know.