I'll prepend this postscript here at the top so it won't get lost in my earlier explanation.
I can reliably produce and resolve the no-permissions problem by simply changing the USB connection type from Camera (PTP) to Media device (MTP). The camera mode allows debugging; the media mode causes the no-permissions response in ADB.
The reasoning seems pretty evident after reflecting on that for a moment. Unsecured content on the device would be made accessible by the debugger in media server mode.
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The device is unpermissioned until you accept the RSA encryption warning on the debugged device. At some point after connecting, the device will ask to accept the debugging connection. It's a minimal security protocol that ensures you can access the device beyond the initial swipe lock. Developer mode needs to be enabled, I believe.
The "no permissions" flag is actually a good first indicator that adb recognizes the device as a valid debugging target. Notice that it doesn't list your other USB devices.
Details at the following and related pages.
http://developer.android.com/tools/device.html
