You can ask me for an Access Database,
but please enter a Subject, otherwise your email will be treated as Spam.
This technique is commonly referred to as
'Entity-Attribute-Values', or 'Name-Value pairs', and is used successfully by SalesForce.com to support a global community of many thousands of Users and to provide User-Defined Attributes,(although not necessarily Hierarchies).
Microsoft has added a Hierarchy ID to SQL Server 2008 which is definitely worth checking out.
SalesForce uses Java and once the appropriate Code Libraries are in place, then this approach scales well and can be used
to provide 'industrial-strength' performance and reliability.
Some details are shown on my Data Model for SalesForce.
You can also try out SalesForce by signing on for a free Trial, and get more details by checking out their Developers
platform at Sforce.com.