What's different about Google Apps?
Four important things. Here's the third:
3) It uses Google's custom-built data storage mechanism, called 'BigTable', instead of a conventional SQL-driven database.

Google has a detailed description here: http://labs.google.com/papers/bigtable.html

That's good news and bad news:

The bad news is that you have to re-write any existing applications that use SQL.

The good news is that the Google 'BigTable' is really easy to use, so it doesn't take long to write software to access your data ... assuming your data structures are fairly simple.

The bad news is that, if your data structures are complex, or if your current application depends on fancy features built into Oracle or SQL/Server such as stored procedures, JOINS, etc - you have a lot of re-writing to do.