EDIT - 06/18/2012 - Thunderbolt solutions are now shipping. See the latest post for details!
After reading through some of the discussion and questions on the Mac SCSI and SAS thread, I'm going to try and explain SAS connectors a bit more clearly. Please understand that this is a simplification aimed at tape use, but should provide a better understanding of how SAS and tape fit together.
There are a number of different SAS connectors in use. However, the two most popular for external devices are the SFF-8088 Mini SAS connector and the SFF-8470 Infiniband connector.
Both of these connectors have the ability to service up to 4 physical devices and carry all four channels available via a port on the HBA to the port on the device.
The design idea is to provide as few cables as possible between a system and an external chassis - such as a JBOD or RAID disk array. The idea being that the multiple channels within a cable would then be separated within the external device which each channel's signals being sent to the proper device in the chassis.
While that design is very good for a disk array chassis, a tape drive doesn't require 4 channels for I/O. Even in the original slower 3Gb/channel, an LTO-5 drive does not hit the performance ceiling of a single SAS channel. Therefore, tape drives only make use of one of the four channels in the SAS cable.
For many, this is seen as a waste of bandwidth since most tape devices using SFF-8088 to SFF-8088 connectivity are only using one of the available 4 channels. To resolve this, some cable companies have produced what are referred to as fan out cables. These cables take a single 8088 connector and split off 2 or 4 SFF-8088 connectors on the opposite end. This means that you can connect 4 tape drives to a single port on your SAS HBA - something that is very useful if you are using a multi-tape library or multiple standalone tape drives.
For example, the ATTO Technology ExpressSAS H380 HBA offers 2 external SAS ports, each supplying 4, 3Gb/sec SAS channels. By default, you could only connect two tape drives to this HBA because the default single SFF-8088 to single SFF-8088 cable only allows you to connect one tape drive to one port, meaning that 3 channels on each port are simply unused.
When you look at cable descriptions, you will see comments such as 4x SF8088 to 4x SF8088 connectors. This should not be confused with a four channel fan out cable. The "4x" in this case indicates that all four channels are transmitted through the single connector.
Hopefully, this makes SAS connectivity a bit more clear. Feel free to post follow-up qeustions and I'll do my best to answer.