Stream Sound from Ubuntu to Android
Ubuntu has built Ubuntu One Mobile for that very purpose. The service is free to try for one month, then $3.99/month (or $39.99 yearly). It's worth mentioning that the paid account gives you 25GB of storage and unlimited streaming for devices running Android 2.1+, iOS 3.1+ as well as Windows XP, Vista and 7 (and there are rates for additional storage).

(For those who do not wish to stream music, Ubuntu One provides 5GB of cloud storage for free.)