Broadway - 8:16 pm on May 18, 2011 (gmt 0)
How about a convention where you could put a mention of links from non-trusted URL's in the robots.txt?
Does a bot look at this file before looking at a page it's indexing?
With this, google wouldn't have to keep track of anything, just access information provided by the webmaster.
I will say, maybe this whole topic isn't really an issue. Maybe other website's linking really can't hurt you. But as long as google operates like a "black box" on this issue, it makes for a lot of uncomfortable webmasters.