Quick: what does the "v" mean in "Prius v", as in the wagon version of the Prius? (Not to be confused with "Prius Five" the top-of-the-line regular Prius, which I take to be a one-up on the Prius Four).
Toyota says that the "v" stands for "versatility" - but maybe that's just a fig.
To me, versatility means the option to change the passenger / cargo ratio at will, so that you can accomodate three children plus their various string instruments, OR your five-member family plus one set of grandparents (or friends). The Prius v you can buy in the US has five seats (what's versatile about that?), so really it should be called the Prius V, as in Roman numeral V, meaning "five", to be distinguished from, say, a Prius VII.
The Prius VII, seating seven, does exist. Only it's called the Prius-α, as in "alpha". (After using Arabic numerals and the Latin alphabet, why not give the Greek alphabet a go?). Launched in Japan in May 2011, the Prius-α has an on-demand third row which accomodates an additional two passengers. In Europe, the same car will launch in July 2012 under the name "Prius+": so here they're going into math symbols, presumably to denote the 5+2 seating.
The seven-seat Prius has a lithium-ion battery (whereas the Prius v sold in the US uses a nickel-metal hydride battery). The average fuel efficiency is quoted to be 62 mpg under the JC08 standard (but that same standard quotes 71 mpg for the regular Prius), so probably 44mpg is a reasonable estimate for the Prius-α, similar to the Prius v.
If you cruised the American Toyota website looking for a hybrid that seats seven, you would find only the Highlander Hybrid: 28mpg and starting at $38,715: nearly 50% more than the $26,550 lowest price for the Prius v.
Wait: you want me to pay $12,165 more for the same number of seats, in a vehicle that does 28mpg rather than 44mpg?
Oh come: let's have the seven-seat Prius v in the US. Go ahead, tack on a Japanese kana (how about ち, "chi", kind of looks like +) - tack on a Russian letter, I don't care, just bring it here!