How to structure database(s) for larger applications?

[eluser]Skinnpenal[/eluser]
Hi!

I'm looking for some views on how to structure database(s) for larger web apps.

I guess 37signal's Highrise or Basecamp could be good examples; large amount of users, and potentially a large amount of data connected to those individual users.

Should there be one database "to rule them all", or should each user (paying customer) get its own database? – OR are there other ways one should go about it?