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After upgrading to Mountain Lion, now when I open the lid to my 2010 13" MacBook Air, there is a 3-5 second delay between when the screen powers on and when the keyboard will respond to key presses in the login dialog box.

This did not happen for me under Lion.

Is this a known bug?

While I believe Macbooks Air did this before Mountain Lion, you seem to be experiencing a resume from standby. All Mac laptops, by default, write the contents of memory to disk when they sleep, but leave the power to the RAM on for a quick resume. Writing the contents of memory to the disk occurs so that you won't lose data if the system loses power.

However, on Macbooks Air and Retina Macbooks Pro, which have built-in Flash memory (which Apple distinguishes from 2.5" SSDs on Macbooks Pro), in order to advertise a 30-day standby time, after an hour the system removes power from the RAM. So, if you leave it asleep for more than an hour, you'll actually be resuming from the disk. It's a lot faster than on a device without an SSD, but it's still a noticeable delay. Apple calls this "standby", and while it's resuming you basically see a still image of the login screen that you can't interact with.

You can disable standby with this unsupported command: sudo pmset -a standby 0. This will cause your battery to drain much more when you leave the system asleep, but it'll make it resume much more quickly.

Apple's documentation on standby mode is here.

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