Meditators aren’t asleep, but they aren’t awake in a conventional sense either. Neuroscience should be able to explain their brain state, and I believe that Antonio Damasio’s theory of consciousness  could help. Specifically, I suspect that meditation shuts down extended consciousness, as he defines it, while leaving “core consciousness” intact.
For Damasio, consciousness is the feeling associated with the relationship between a perceived object and the perceiving organism. Damasio argues that consciousness consists of two levels: core consciousness and extended consciousness.
Core consciousness provides the organism with a sense of self about one moment (now) and one place (here). It arises from moment to moment, and is constructed out of the pulses of awareness generated by changes in objects and bodily states. It’s a very simple biological phenomenon, and is not exclusively human; it does not require language.
Extended consciousness provides the organism with an elaborate sense of self that’s based on an extensive memory and a rich sense personal history and anticipated future. It’s wrapped up with an identity and an elaborate sense of self, and is intertwined with language. Extended consciousness is built on core consciousness. Patients with impairments that shut down extended consciousness continue to show core consciousness, but once core consciousness is lost, extended consciousness also disappears.
I suspect some meditation techniques  are de-activating extended consciousness, leaving only core consciousness functioning. Many of the topics in meditation practice match Damasio’s description of core consciousness. Practitioners are advised not to verbalize their experience, but simply to be aware of sensations from moment to moment (cf. Damasio’s insistence that core consciousness is pre-linguistic). They are said to become aware that everything is constantly changing (cf. Damasio’s pulses of core consciousness). The sense of a persistent self fades away. However, there is still a sense of consciousness; meditation is not sleep. Hence, in Damasio’s terms, there is still second-order awareness of the relationship between the organism and the sensations it is experiencing.
This hypothesis immediately suggests some questions:
- Damasio is quite specific about which parts of the brain are responsible for different states of consciousness. One should be able to use fMRI of these regions in experienced meditators to test the hypothesis that meditation shuts down extended consciousness while leaving core conscious functioning.
- There is a growing body of evidence of the beneficial health effects of meditation. Can one connect differential activation of different kinds of consciousness in Damasio’s model to specific benefits?
- Do higher states of meditation lead to partial shut-down of core consciousness, in the same way that “introductory” techniques like anapana shut down extended consciousness?
 Antonio Damasio, The Feeling of What Happens: Body and Emotion in the Making of Consciousness (2000)
 There are many kinds of meditation. I have a rudimentary knowledge of the approach known as insight meditation, aka vipassana. See wikipedia for more: http://en.wikipedia.org/wiki/Buddhist_meditation. The most obvious candidate for meditation practice that suppresses core consciousness is “anapana”, a kind of tranquility meditation that aims to concentrate the mind.