I just uploaded UKSoundFileRecorder 0.1, a nifty little Cocoa class that you can use to record sound from the system's default sound input source and have it written to a file. It should work with key-value-observing (and of course key-value-coding), and supports notifying a delegate of start/stop recording and to give it periodic updates on the duration of the recording.
It can be configured to create different formats and file types, though I've only used AAC so far, so let me know if there's problems with anything else. It uses CoreAudio behind the scenes (my "inspiration" was Apple's RecordAudioToFile sample code), so obviously it has the same limits as CA regarding output formats.
Get it from the Source code page while it's hot.