Here is what I found:
http://64.233.179.104/search?q=cache:zoyxKHzSNEMJ:www.skyhero.com/bininst.pdf+brainwave+of+meditation+alpha+stage&hl=en&client=firefox-aThe four major brain wave divisions:
Beta: Beta is associated with normal, waking consciousness while our attention directed towards our external environment. They are also present during stress and anxiety. Beta waves oscillate between approximately 14 and 30 Hz (cycles per second).
Alpha: Alpha occur when we are relaxed and not thinking about anything in particular. Alpha waves are also associated with a calm and focused mental state. Alpha waves oscillate between 8and 14 Hz.
Theta: Theta is a state of deep relaxation and is often associated with bursts of creative insight; sleep learning and vivid mental imagery. It is also found in more advanced Meditators. Theta waves oscillate between 4 and 8 Hz.
Delta: Delta is the slowest of brainwave activity and is found during deep, dreamless sleep and sometimes in very experienced Meditators. They oscillate between .5 and 4 Hz.
NOTE: The brainwave states are an indication of the most dominant patterns during
any given mental activity. The brain is a dynamic, living organ. During these dominant
patterns, other frequencies still exist and there is no exact, single frequency that our
brain operates on.
Many more articles on Meditation and the Brain:
http://www.washingtonpost.com/wp-dyn/articles/A43006-2005Jan2.html &
http://www.crystalinks.com/medbrain.html