This is also a good web page...
http://www.wlu.ca/~wwwss/greek/parents.htm
it is quite good for parents who might not know anything about Greek Life or who might have all the negative preconceived notions about it...
I think your idea about a letter to parents is a great idea....it will make them feel that they are involved and will hammer home the idea that Greeks realize that their members have family members who love them, care about, etc.
One thing that you might want to consider is the letter might be great for "younger pledges" (age 18-21).....If you have a pledge who is 26 and has been living on their own since age 18, then a letter to the parents might make them feel a bit "babied". So I think it would be wise to take each new member's situation into consideration.