I've been holding off on this comment until ABC and CBS reported their latest results, since they are the two pollsters who have shown Nader having the greatest effect on the race. But there does now appear to be a definite trend in Nader's effect on the polls.
http://tis.goringe.net/pop/nader.html
While Nader's share of the vote has not changed significantly, his impact on Kerry has. In March, the inclusion of Nader in the poll saw Kerry's vote drop by 3.1%; in April it was 2.8% and in May it is down to 2.3% (so far). The share that Nader takes from 'other/don't know/won't say/depends' has risen from 0.1% to 0.9%.
Nader still draws only 1.1% from Bush, so he still has a net negative impact on Kerry, but this impact is getting smaller.
Results based on a total of 41 polls.