UPDATE 2/23/2011: See the latest test user documentation.
You can now make officially sanctioned test accounts
Full details are posted on the wiki here:
but here is the short version of the important parts:
A few important things to note:
* Test accounts won't be able to see real Facebook users and vice versa. You can make an existing account a test account, but if that account is friends with a real account, each user will disappear from the other's list of friends.
* Our customer support team won't disable test accounts for being fake, but test accounts may be disabled for violating any other reason that a real account would be disabled.
* Only real accounts may be listed as application owners. In other words, you may not list a test account as the developer of an app.