The "or maybe this" was pointing to the www.coastaltech.com website, which has a the "cruise control stick" mod for getting EV mode out of U.S. Priuses.

As far as the wisdom of running in EV mode, well you would have to peruse the Prius boards for non U.S. owners who have first hand knowledge of that to get the real scoop.

Here is a doc that explains some of it:

PDF file for EV Button installation

Here is another link to a Prius forum that discusses it:

PriusForum

Google this string:

prius +"EV mode" +button

and you will get tons of relevant hits.

I would doubt that Toyota engineers would have included the EV mode as standard had there not been a good time to use it.