An informative and readable article.
However, if I may make one correction: In describing eSATA and SAS external options, you say that SAS can be used to hook up arrays of drives while eSATA allows only one drive per cable. This is inaccurate. eSATA multilane is available now, and I use it daily. The Sonnet Fusion 500P is an example of an external eSATA device that hooks up 5 hard drives to your computer using only one eSATA cable. It does not present the drives as one array either - you see 5 hard drives in the Windows device manager, and they are hot-swap also.
Getting multilane eSATA to work with Linux? That's another challenge entirely...
Multilane does not allow more than one drive to be connected per SATA channel. What you're thinking about is probably a SATA Port Multiplier. These devices will allow up to 15 drives to be connected per SATA channel, however, most Port Multipliers only have connections for 5 Drives at a time (or less). Multilane is only a means to 'bundle' more than one SATA cable, into one large cable (IE, it is externaly one large cable, but inside are 4 cables for 4 HDDs, or 4 SATA connections, inside the enclosure / connected machine, there are still 4 single connections).
Port Multipliers also need a compatable HDD controller, to enable FIS based switching, which is usually a SIL 3132 or equivelent (SIL 3114, and the newer SIL 35xx controllers, I beleive). What FIS switching does, is enable the drives to work at full capacity while sharing the same SATA connection. Without it, you could not run RAID ( unless perhaps in software, and this would offer terrible performance ). Read about it
Here . Regardless, one drive only could be accessed at a time using using a Port Multiplier without a compatable HDD controller chipset.
SAS is much better, with a good controller card, and multiple expanders, you could hook up to 128 drives per controller. SAS is also very expencive, but if speed is what you need, SAS is the only real way to go. Also, finding good hardware for SAS can be a bit hard right now, and getting expanders, without buying an expensive rack, or 2.5 formfactor drive tray assembly is next to impossible ( if not impossible outright ). Now, when I say 'expencive', we're talking $200 for a single 15k RPM 36GB Seagate Cheetah, at least $300 for a cheap controller that only offers RAID 0,1,10, plus if you plan on using the newer mini 4x 2.5 drive from 1x5.25 drive racks (with expander), add another few hundred into the mix). So, we're talking ~$500-$700 for a single drive ! The newer SAS Cheetahs that offer 300GB storage space, $1200 for a single drive only !
Port multipliers are an OK solution for the home envoirnment, but keep in mind that the RAID achieved handled using this technology is currently software only ( Even though, there are dedicated PM enclosures out there that handle the hardware internally, just dont expect them to be cheap). Also there are other solutions out there, Just need to google around, ask a friend on IRC, etc, to find out what is out there
[EDIT]
I probably should have mentioned that using a SAS controller, you are not 'obligated' to use SAS drives, but in fact, you CAN use SATA drives with it. This means, with a SAS controller, and Expanders, you could also have up to 128 SATA HDDs connected to it. Pretty nifty technology . . .