it has been determined that rel=nofollow (when used internally) can actually ‘evaporate pagerank’ (reference bruceclay.com) They basically say that if a page is PR10 with 10 links, and 5 of them are rel=nofollow, that still only 1 point of pagerank is passed to the 5 “do follow” links (instead of 10/5=2 points).
My question is - does anyone know if using the content=nofollow tag in the robots.txt file does the same thing?
No. These are two different tools that do two different things. The meta nofollow is an instruction to the search engines to not crawl any of the links found on that page. The only way those destination pages will be crawled is if they are linked to from other pages/locations.
So it is more restrictive than link-level rel=nofollow, in that not only is no PR passed at all from that page, but the links on that page aren’t even added for discovery.
Some more reading: