TMZ just broke the news! Interesting timing since his ex-wife Gwyneth Paltrow just got married!
Blue and pink balloons were tied down in arches ... as their celeb friends arrived. Dakota's folks -- Melanie Griffith and Don Johnson -- walked in, along with Sean Penn and Julia Roberts.
Looks like Don was last to arrive ... because about 10 minutes after he got there, the string of blue balloons were released into the sky. So, all signs point to them having a boy, unless someone screwed up the release.