I also would add, that if this is the ONLY contact over the years or the only letter you expect to have, the letter will carry alot more weight with your daughter than if her bmom has been in more frequent contact with you and your daughter over the years.
I think in this situation, as a minimum, a series of letters that are age appropriate is probably the best (other than visits where bmom can explain her reasons to your daughter in person), so multiple letters is certainly better than only one letter to be given in adulthood.
Your daughter does have a right to know those details at some point if she wishes but its not appropriate for her to know those details under the age of probably 18 or so. But before that age she is going to have alot of questions (probably starting at age 3 or 4) about her birthmom and the cirucumstances surrounding her birth and relinquishment. If you are unable to have an ongoing relationship with bmom, I would suggest asking her to then write the series of letters. Maybe one a year for her birthday written to her at that age. Obviously the information acceptable or intersting to a 3 year old is significantly different than information interesting to an 8 year old etc. Most books etc out there recommend that the child should know all pertinent information (like being conceived through sexual assault) before puberty - but she doesnt need to be told the details of the sex act etc unless she desires that in adult hood.
Some good books relating to this topic are
"To Bless Him Unaware" which specifically deals with children conceived through rape as well as "Telling Difficult information to Foster and Adopted Children" (or something like that ...
)
You might also want to get ahold of amom2two who is a member here. She recently adopted a baby that was conceived through rape and she has lots of information that I know she would be happy to pass on.
Good luck and let us know what you decide
Jen