I don't know the math well enough to comment on whether Nate is getting it right or not, but he makes a good case for why his method is superior to other poll aggregation sites like Real Clear Politics or Electoral-Vote.com.
Anyway, Silver's fellow stathead Tom Tango has this comment on Nate's analysis on his site today:
Nate has Obama with an 89% win expectancy as of today. Bottom of the 9th, 1 out, down by 2 is a 96% chance of winning. Down by one though is a 90% chance. So, that’s where we are in the race. Bottom of the 9th, 1 out, down by 1 run, McCain batting, Obama pitching.Whether you're for Obama or McCain, you gotta love it.