Apple, following their announcement at the It's only rock and roll, but we like it special music event, has just posted iPhone OS 3.1, promoting it's Genius App Recommendations, Genius Mixes, Downloadable Ringtones ($1.29 - ouch!), and support for iTunes 9's App Management feature.
Just plug in your iPhone, hit Update, sit back, and enjoy the new hawt. (Jailbreakers and unlockers should hold off unless you want to lose your jailbreak/unlock.)
For more on iPhone 3.1, see TiPb's iPhone 3.1 feature page.