Just keep in mind that HipHop will not magically accelerate your SQL queries and will, in no case give you the performance of using Cached Static HTML.

Preliminary estimation (by grepping the use of eval) indicates that it does look promising:

foo@bar:/var/www/hiphopphp.org$ grep -r eval\( . | grep php
./wp-admin/includes/class-pclzip.php://      eval('$v_result = '.$p_options[PCLZIP_CB_PRE_EXTRACT].'(PCLZIP_CB_PRE_EXTRACT, $v_local_header);');
./wp-admin/press-this.php:              var my_src = eval(
./wp-admin/press-this.php:                      var my_src = eval(
./wp-admin/press-this.php:                                                      eval(data);
./wp-admin/import/blogger.php:                                          eval('this.status='+text);
./wp-admin/js/revisions-js.php:eval(function(p,... [CUT]
./wp-includes/class-json.php: * Javascript, and can be directly eval()'ed with no further parsing
./wp-includes/functions.php:            if ( doubleval($bytes) >= $mag )
foo@bar:/var/www/hiphopphp.org$

NOTE: This does not cover any popular plugins, some of which might be using eval.