As I can't even get to Google Plus from work this seemed a tad silly, so I went looking for a solution and discovered that adding plusone.google.com/u/0/_/+1/fastbutton to my Adblock Plus filters** suddenly reduced the memory that Google Reader was taking up from 400MB to 9MB. Where it then stayed for the rest of the day.
*Except for when Firebug is running. That thing eats memory.
**I've never added a filter before, ABP seems to be pretty awesome at spotting ads.
Original post on Dreamwidth - there are comments there.