Alternatively: detect when a URL is pasted and remove the placeholder http:// only then. I certainly type URLs sometimes and would find it disorienting if the http:// just disappeared when I started typing.
But how would I know it's going to do that? I'm gonna delete the http:// if I've already got an http:// on the clipboard because I can't be sure it behaves unlike every other text box I'm used to using.
If it disappears on focus, it's before you start typing (or pasting).