I recommend you use the jQuery Placeholder polyfill by Mathias Bynens.
- It supports IE6+
- It is 2k minified
- It does its own feature detection (Modernizr isn’t needed)
- The code is straightforward. The jQuery selection will vary depending on your selector
- It is the one polyfill recommended by HTML5Please and is listed on the Modernizr Wiki you mentioned
- It doesn’t execute the code if it isn’t needed