Why wouldn't you use the placeholder, attribute, then detect support, then use a fallback if needed... Something like:<p>function hasPlaceholder() {
var input = document.createElement('input');
return ('placeholder' in input);
}<p>should do the job to detect support.
Not sure why... in-field labels are a better approach. <a href="http://bit.ly/r4vVAa" rel="nofollow">http://bit.ly/r4vVAa</a> Plus the placeholder attribute is the long term.