Now, a couple of caveats here:
1. Third-party candidates almost always poll better than they ultimately do in the ballot booth, especially early in an election cycle.
2. Zogby mentioned Nader by name in its telephone interviews, whereas most other pollsters have not. There is not necessarily anything wrong with mentioning Nader specifically, but sometimes the mere power of suggestion is enough to get a candidate a couple of percentage points. If instead of using Nader, you asked instead: "If the election were held today between Barack Obama, John McCain, and former Minnesota Vikings quarterback Fran Tarkenton, who would you vote for?" -- Tarkenton would probably get a point or two.
3. It's Zogby
Still, Nader's numbers are presumably being driven upward by the current Democratic pie fight. I don't particuarly believe that Nader is likely to poll at 5 or 6 percent when it's all said and done, but it's plausible that some of those voters will not turn out -- or will require time and energy to bring back into the fold that might otherwise be spent on more traditional 'swing' voters.