Design question: Scalability and tens of thousands of tables?

Hi everybody.  Quick question:

I'm designing an application that will allow users to create custom objects
on the application level.  A custom object can have zero or more attributes.
Attributes can be one of 5-10 types (ex. String, Number, List, Date, Time,
Currency, etc...). This will allow users to track anything exactly as they
want.  My first thought on how to structure this is to make a custom table
in the db for each custom object.  The attributes would map to fields and
everything would work just dandy.

The problem is I am very concerned about scalability with having a different
table created for each custom object.  I want to design to site to handle
tens of thousands of users.  If each user has 3-5 custom objects the
database would have to handle tens of thousands of tables.

So it would appear that this is a broken solution and will not scale.  Has
anyone designed a similar system or have ideas to share?

Thanks,
Zack