If you're a Boost Mobile customer, get ready for some major changes. Optus is shutting down the youth-oriented prepaid brand on January 20, 2013.
If you're an existing Boost customer, you'll be able to continue on your current plan after that time, but you'll be treated as an Optus customer. Boost has always been owned by Optus and used its network, so that's not likely to make a huge difference to your experience.
Optus' announcement says it wants to consolidate its prepaid customers onto a single brand. Despite, it will apparently continue to operate Virgin Mobile, which offers both prepaid and contract options.
In the current market, Boost isn't especially competitive, and its very high rates for casual data and use of 10MB increments have made it increasingly irrelevant. Its $40 per month unlimited calls and 3GB of data deal isn't terrible value, but Amaysim offers more data (4GB) on the same network for the same price.
Update: As a commenter helpfully points out, Telstra plans to pick up the licence for the Boost brand. No word yet on how customers who want to switch from Optus Boost to a new Telstra Boost account will do so, but I imagine details will emerge before January.