If people who want to vote for Nader are denied the opportunity - will they even bother to show up at the polls? If they don't, won't this hurt the Democrats in congressional races?
It's probably not of any great consequence. For one thing, people inclined to vote for Nader aren't that many. They were less than 3% in 2000 and will be less than 2% (probably less than 1.5% in 2004). Why people give any credence to early-cycle poll showings of 3rd Party candidates when they merely serve as a proxy for None of the Above is beyond me.
Furthermore, the areas where Nader's support is concentrated are already heavily liberal areas where the House incumbents will win by 60%+ margins, often 70-80%. In addition to that, those states do not feature any competitive Senate elections this year where one may argue that Nader voters might tip the balance - arguable exception, Florida.
I personally think Nader is inconsequential as I've often said before. The protest vote is routine and it doesn't really matter who picks it up - the names are interchangable. That's why you see some polls where Nader actually slights Bush more than Kerry (which leads some people to foolishly dismiss them). A third party contender who can break out of the protest-vote gutter is always self-evident (1912, 1968, 1980, 1992) - Nader 2004 isn't it.