There are currently two main GNU/Linux distributions for business: Red Hat and SuSE. So it is perhaps no surprise that Sun, which badly needs to start pushing the free operating system if it wants to play in world of open source enterprise stacks, should choose something else entirely - Ubuntu, to be precise.
This makes a lot of sense: in doing so, it guarantees that it will be the senior partner in any enterprise developments, and ensures that it is not drawn into the orbits of IBM (with Red Hat) or Novell (with SuSE).
It also has bags of potential in terms of branding. Ubuntu is famous for its "I am what I am because of who we all are", as well as its tasteful mud-brown colour scheme. Now, imagine an enormous, burnished sun rising majestically over the rich, dark pullulating earth....
Update 1: Interesting interview with Mark Shuttleworth on the enterprise-level Ubuntu.
Update 2: Further confirmation of the alliance: Ubuntu running on Sun's Niagara servers.