TUAW claims Apple will release a new Mac mini lineup at MacWorld Expo 2009 next week. The Apple rumor site claims the new Mac mini will get an enclosure with aluminum and black plastic elements, and a SATA optical drive that's hidden under a lid.
First, the internal optical drive is changing to a SATA optical drive that can be swapped out for a second SATA hard drive, probably as a build-to-order (BTO) option. For those using Mac minis as departmental / small business servers, having a second internal hard drive adds a great deal of utility to the mini in a server role; RAID 1 mirroring becomes simple. Companies like MacMiniColo.net that colocate thousands of Mac mini servers will save rack space as external drives will no longer be mandatory for backups.
The second detail is that the new Mac mini won't be made of topped with white plastic like the current model seen above; instead, it will follow Apple's new design meme by using black plastic and aluminum. No more smooth sides, either -- the new mini has a "lip" similar to the Time Capsule. The optical disc slot is hidden under the lip so it won't be obvious in case the owner chooses to have two hard drives instead of a hard and optical drive combo. The lip also provides cooling, so the vents on the back of the mini are no longer needed.