Came here to post a summary but you did a better job than I could.

   > A few companies/research groups are working on bridging 
   > the gap between the extremes (unencrypted/fully 
   > homomorphic encryption) in order to come up with a good  
   > compromise between security and cost.

I just wanted to add that plain text vs homomorphic encryption is indeed a security and cost trade-off.

Fully vs partially homomorphic encryption is a trade-off between supported operations and cost. Fully homomorphic systems allow arbitrary computations while partially homomorphic systems do not, hence the lack of support for standard SQL.