[jgb@etree]

Well, I can confirm that the issue is related to having saved profiles for multiple networks that are available at the same time. When I keep all off the WiFi profiles (except for the one I want to use) disabled, the WiFi is stable (tested on 6 different networks). As soon as a second (or more) saved profile is marked active, and the Playbook gets within range of 2+ saved networks, it starts switching between them every 30 seconds or so. After 4-5 minutes of this, I can't connect to any network until I disable wifi & re-enable it.

I can deal with the workaround of having to manually enable/disable wifi profiles as needed, but would prefer not to. If anyone finds a solution (or better workaround) I'd love to know.