Much ado was made of Derek Sivers's choice to migrate CDBaby from Ruby to PHP. Although I think CDBaby itself is noteworthy, this particular decision isn't. A similar decision was made when Friendster migrated from Java to PHP. Derek's motivation seems to be more about maintainability than performance, but both represent a shift from heavyweight frameworks to a lean, mean, fat-grilling web machine. Language devotion and hype are the only things that make such decisions seem surprising or even noteworthy.
I was interested to learn recently (via James McGlinn) that del.icio.us is switching to PHP (and the Symfony framework). It represents yet another migration to PHP, but this time it's from a mature, lightweight solution to a framework. Perhaps the fact that Yahoo Bookmarks uses the same framework has a lot to do with the decision, but regardless, a migration to a framework seems more noteworthy than a migration from a framework.
With this migration, PHP now powers most of the darlings of Web 2.0:
At The Future of Web Apps, I gave a workshop on web application security. I was pleased to learn that the vast majority of attendees were using PHP. Not because I cared whether the future belonged to PHP, but because all of my examples were written in it. :-)