This is a good question. It is unreasonable for a whistle-blower to expect total anonymity to last. There are three reasons for this.
First, we live in a world of constant media coverage. And with the Internet, things are even worse. For this reason, it will be hard to be anonymous.
Second, often times whistle-blowers will have to testify to fix the problem at hand. This fact alone will make it impossible to be anonymous.
Finally, people who seek to hide what they are doing will seek to single out the whistle-blower and make that person not talk.
All of these reasons suggest that it is not reasonable to expect anonymity. Mostly likely word will get out.