I don't think that's authentic. The overall look is wrong, and to be more specific if you look very closely at the autograph you can see that the first part, (the part that look like a W) is separate from the second part. In any authentic examples I've seen these two part are always attached, and written without the 'pen' leaving the paper. And the vertical line is always done last, in this autograph it's seems attached to the first part.
Don't know how to explain it better. But here's an example of an authentic Johnny Depp autograph.
http://cgi.ebay.com/Johnny-Depp-Pirates ... 2a05fa8098
The seller has a few other Johnny Depp's up for sale, all authentic, and if you compare these with yours you should see what I mean. Even though the signatures vary a bit, some are more rushed then others, the basic make up of them is the same.
Just my opinion. :neutral: