SPMediaKeyTap abstracts a CGEventHook and other nastiness in order to give you a relatively simple API to receive media key events (prev/next/playpause, on F7 to F9 on modern MacBook Pros) exclusively, without them reaching other applications like iTunes. SPMediaKeyTap is clever enough to resign its exclusive lock on media keys by looking for which application was active most recently: if that application is in SPMediaKeyTap's whitelist, it will resign the keys. This is similar to the behavior of Apple's applications collaborating on media key handling exclusivity, but unfortunately, Apple is not exposing any APIs allowing third-parties to join in on this collaboration.
For now, the whitelist is just a hardcoded array in +[SPMediaKeyTap defaultMediaKeyUserBundleIdentifiers]. If your app starts using SPMediaKeyTap, please mail me your bundle ID, and I'll include it in the canonical repository. This is a bad solution; a better solution would be to use distributed notifications to collaborate in creating this whitelist at runtime. Hopefully someone'll have the time and energy to write this soon.
In Example/SPMediaKeyTapExampleAppDelegate.m is an example of both how you use SPMediaKeyTap, and how you handle the semi-private NSEvent subtypes involved in media keys, including on how to fall back to non-event tap handling of these events.
SPMediaKeyTap and other CGEventHooks on the event type NSSystemDefined is known to interfere with each other and applications doing weird stuff with mouse input, because mouse clicks are also part of the NSSystemDefined category. The single issue we have had reported here at Spotify is Adobe Fireworks, in which item selection stops working with SPMediaKeyTap is active.
SPMediaKeyTap requires 10.5 to work, and disables itself on 10.4.