Do Clean URLs Still Matter?

Keyword stuffing, uppercase, lowercase, hashbangs, shebangs, query strings, underscores and dashes, websites, web apps, oh my! URLs are something I have been researching and reading about a lot lately and I am a bit shocked by the thoughts of some very smart folks.
Google +1 Button

Google +1 Button Validation Error

Since Google recently added +1 Metrics to Webmaster Tools and Social Engagement to Google Analytics, I decided to add the Google +1 button to my website. All went well until I ran my site through the W3C’s HTML validator. The new button caused my previously valid site not to validate.