Another vote on this. Annoying "issue".
I had the same issue on my iMac and it was really annoying. I found that un-checking the box that reads "automatically connect to networks" (or something to that order) helped fix my issue. It may work for you too!
Re:Mac OS X 10.8.1 is out.
by MattRogish on Thursday August 23, 2012 @02:27PM
(#4424035)
I have this problem as well; it's my most frustrating experience with Mt. Lion so far.
Where is that? I tried Spotlight searching for "automatically" and "networks" and couldn't find it.
If you just go to Network Preferences you should see it.
Re:Mac OS X 10.8.1 is out.
by captainknz on Thursday August 23, 2012 @04:07PM
(#4424544)
I noticed a simple solution: Wait for the mouse pointer to show, then you're good to log in
Mountain Lion - open lid, push first character of password, no response? keep trying again until asterisk appears, then type rest of password.