Is depends on the meaning of downtime. If you look at if from the user point of view of not people not being able to do their jobs and make money for the company, then I would say that most of our downtime is caused by application issues:

Stored proc updating a table and blocking 300 users for a couple of hours at the peak busy time.

Overnight DTS package job failing to complete and leaving the database not ready for business users at start of day.

Production application with a connection string pointing at a database on a development server, so all users could not log in after we shutdown an old development server.

Purge stored procedure had logic in the where clause wrong so it deleted only the data that was needed and left only the unneeded data.

Application running queries in a loop that was supposed to refresh every 5 minutes, but was running 1000 times per second and making SQL Server unresponsive

etc.

Of course we have hardware and network related problems, but developers are the ones we depend on to create the really big screw-ups.