As I predicted in Wither Thunderbolt? a few weeks ago (“Thunderbolt will be expensive for a year or so”), Mac Rumors writes that a Thunderbolt interface will cost $100 or, which is more than many hard drives themselves cost today. I confirm that figure as accurate based on my own sources also.
In other words, an external quad-interface drive costing $139.99 would have to be priced in the $240 range were it to have a Thunderbolt port. This might be acceptable for drives in the $500 on up range, but it’s a non-starter for 99% of the market.
New Thunderbolt chipsets in 2012 will bring that overhead cost down drastically, but until then you’re likely to see the Thunderbolt interface only in high-end gear such as 4-drive RAID boxes and the like.