Media Temple launched their latest offerings today along with a redesigned site. After logging in, current (ss) users will see a new button allowing them to migrate to the new (gs) platform. The (gs) runs at $20/month for 100GB of storage and a terabyte of bandwidth. I'm not sure how the pricing works for those that have already purchased the (ss) a while back.
As Michael Arrington noted, there is not much competition for Media Temple's Grid-Server. The nearest competition charges five times what (mt) does. The load on the (gs) is spread out over many servers, not allowing for traffic spikes to negatively alter performance. However, if your site does receive a major spike in traffic you will be charged for overages in bandwidth and CPU usage, that is if you can consume anywhere near 1 TB of bandwidth.
Kudos to Demian and the whole (mt) crew for a job well done. Be sure to check out the TalkCrunch podcast.