Truncate product description when used as meta description

I ran into some nastiness on a product with a very verbose description, which was being used as the content of the meta description in the header. I googled this up for a reasonable limit:
http://www.seomoz.org/learn-seo/meta-description

It says 155, so this truncates at 160 with `separator => ' '` so we don't truncate mid-word and truncations should end up with a max of *approximately* 155.