Quite simple, no. The client application needs to know the key for authentication with Bing Maps servers, and since the Javascript code cannot be completely confused, it will always be possible to extract your key from the code on the client side.
There are various ways to hide your key from a person’s eyes by looking at your source code (for example, those discussed in the messages listed by Sandeep), but they are effective against only the most casual users - it’s trivial to use tools like Firebug to reveal the key for any user so motivated for this.