This is the first time but is also not, there have been duets with Siri too in past and many talks with it but this is something damn impressive. The Music Hack Day 2011 competition is going to take place in London for which Hacker Robert Boehnke by using the Siri Proxy made a  RAP of Hypnotize by Notorious B.I.G by Siri. The Next Web (TNW) claims

This hack, accomplished for the Music Hack Day 2011 competition in London, has Siri laying down some serious flow in a rendition of Hypnotize by Notorious B.I.G.

 Sir proxy has already spread around like a flu, now every one can do any thing with it even who is a beginner, no a professional.
Robert Boehnke told about his work on the rap

SiriProxy is used to intercept the communication with Apple’s servers. Based on your song request, Notorious Siri then sends your choice of Notorious B.I.G.’s Hypnotize and an a-cappella rendition of Queen’s Bohemian Rhapsody to the device (the latter requiring 4 iPhones 4S).

Siri’s speech synthesis is synced to the beat using the timestamps obtained from the Echonest API which were then manually tweaked, to smooth out delays in the text-to-speech engine.

We have embedded the Siri RAP and for your fun and convenience because after listening to the RAP many of the folks will head to YouTube to search for the original song