By now you'll all have read the RC post: Decrease in Atlantic circulation? (see, thats where I got my question mark from); which is about Bryden et al. in Nature. RC, of course, has a nice science analysis; I just wanted to compare it to the Grauniad story: Alarm over dramatic weakening of Gulf Stream. Now the headline is nonsense, because the Nature paper itself sez: the northwards transport in the gulf stream across 25 oN has remained nearly constant. Something more complicated is going on (return flow shallower, hence warmer, hence overall heat transport N is less because more is coming back S), which I'm not going to explain because (a) its over at RC and (b) I haven't read the paper properly yet (I look forward to doing so tomorrow during my long flight). It looks to me like it was too complicated for the Grauniad sci writers.
There are I think various caveats to interpreting this: most notably (as RC notes), that if this really has already happened, you might expect some signal in the SSTs which doesn't seem to have been seen.
Also, the Nature article and the Nature commentary are a bit selective in their reading of GCM results to support this.
[Update: also see James Annan's take and wise words re Nature]