Could someone compose a quick precis for me on the prepaid calling card market? 
1. Different types of cards available and strengths/weaknesses
2. Major calling card companies and market share data
3. Size of market, growth potential and history
4. Demographics - who uses them and where?
5. Costs and profit margins

I can not give you all the answers, just partial response.

1) You can segment prepaid calling card market by
-a) virtual (sold over the internet, you do not get a physical card)
and physical (you buy a piece of paper of plastic). Virtual have lower
marketing costs, require a buyer to have a credit (PayPal or credit
card).
-b) rechargeable and non rechargeable. Rechargeable enable you to use
the same PIN after recharging
-c) non fee and various fees (non fee is usually more expensive by minute)

2) Major players
-a) Nobelcom http://www.nvcards.com/ virtual calling cards, physical calling cards
-b) Cloncom http://www.prismlongdistance.com/ virtual calling cards,
recharge for cellular phones
-c) Tel3advantage http://www.3telcom.com/ virtual calling card dialaround

3) NA
4) International callers, students, ethnic markets
5) cost from about 1c/min for state to state in USA and about the same
1c/min USA to other countires. It is usually more expensive when
calling back to USA.

find some data on the size of this market (annual sales) and trend
data, also find out what kind of rates you generally get from those
cards in the bodegas/convenience stores, and I will mark this as an
answer. Thanks for your research.

1. Calling cards classification is very simple:
	On the means of connection: 
 	   Connection via phone, including cellular, home, office, dorm   
and  payphone.
       	   Connection via computer (PC to Phone).
	On the extra fees: 
           Fees are charged (connection fee, maintenance fee, etc). 
           No fees.
	On the PIN-code: 
           Temporary PIN-code. 
           Rechargeable PIN-code. 
           PINless.  
2. Major companies selling calling cards on-line: Cloncom.com,
MasterBell.com, PayTone.com, CallingCards.com.
3. Size of market is about $7 billion per year. Growth is nearly $1
billion per year. History of a calling card has begun from the
vandalism of payphones which was caused by a shortage of coins in
Italy in 1975. The first magnetic strip phone card was made so people
wouldn't need to use coins to make a call.
4. Cheap international calling cards are extremely popular with
college students, truck drivers, travelers and international visitors.
5. Cost from about 1c/min within USA to 30c/min to other countires. In
just over the last half-year, for some destinations the rates on calls
with phone cards reduced more than on 30 %. For details and purchasing
calling card on-line go to International Phone Card Review
http://www.evocallus.com.