Seeking advice on building an application based on LDAP

[Ron Wheeler <rwheeler@artifact-software.com>]

I am designing an e-learning system that will server many (~100) 
organizations that each have tens of thousands of employees.
Each organization can have multiple organizational levels.
Each organization can have course catalogs at each of the organizational 
units and each employee will have a personal course catalog and some 
limited history about courses taken, marks received, certifiactions 
achieved and courses in progress.
The organizations may have their own administrators at each level who 
will be able to sign students up for courses.

The master course catalog in LDAP will have minimal information in it 
with pointers to external files/database entries (MySQL)

The client will interface to this through Web Services so it will never 
be exposed to the internet directly. I am hoping that LDAP will be easy 
to access from Web Services.

It seems possible and looks like it might actually be a "good" idea but 
I would like to get some expert opinion.

From my reading, it appears that LDAP can be used to hold all kinds of 
information that has the following characteristics
1) few updates, many searches
2) naturally hierarchical
3) needs scalable repository

I have found some very good help on the web so far.
The IBM Redbook series was very interesting and helpful. It was a bit 
confusing because of the customization that they had done to the schema 
stucture which they treated as part of the standard LDAP functionaility.
"Understanding and Deploying LDAP Directory Services" by Timothy A. 
Howes, Mark C. Smith, Gordon S. Good looks like a good book with some 
sections on customization.

Some of the advantages of using LDAP are:
1) simple directory style navigation
2) intrinsic replication and proxy capability for scalability
3) standards based but customised to the point of unrecognizability
4) single database for authentication and full user profile
5) distributed administration built-in to LDAP design
6) natural ability to handle multiple organizations and multiple 
organizational units within each organization.

The alternative is to go all custom with everything in a series of MySQL 
databases.

From this limited thumbnail, what advice can you give me?

1) Am I on the right track?
2) Does anyone know if anyone has ever done this and created a schema 
that can be used?
3)  If I use LDAP, what kind of problems am I going to have? Besides the 
usual errors
4) Any design guidelines that you have found useful in the past.
5) Any good resources for customization of LDAP schemas.
6) Any good design tools for planning and documenting LDAP structures
7) Should I care about the backend that LDAP uses?

Thanks
Ron