Update August 4, 2010:

A third party developer has released the free “Locale gtalkcontrol Plug-in” on the Android Market which implements this feature request. This plug-in uses private APIs, so there are no guarantees that this plug-in will continue to work in the future however. https://market.android.com/details?id=com.dattasmoon.gtalkcontrol

Unfortunately, apps like Locale cannot modify the notification settings of Google Talk or any other individual app. Locale can only change the system-wide notification settings (e.g. disable all notifications).

The only ways to accomplish this would be:
1. Google creates a Locale Plug-in for Google Talk. (This would be similar to the K9 Mail Plug-in)
2. Some sort of Locale plug-in to sign you out of Google talk. But I’m not sure if this would be possible or not because I haven’t looked at the Google Talk APIs.

I definitely think Spotify needs such a feature. At the moment, the clearest path to completing this request is to direct it at Spotify.

This would be difficult for two forty four a.m. or any other developer to implement, because Spotify doesn’t offer any APIs to make this happen.

So the burden is on Spotify to do one of three things:
1. Spotify should make their client smarter, and pause playback when headphones are disconnected
2. Spotify could create a Locale plug-in, so that Locale (and other apps that implement a Locale plug-in) can control Spotify.
3. Spotify creates an Intent-based API for their Android client.