No one is forcing anyone to vote for Nader. It is insulting the intelligence of eligible voters to imply that they are being somehow tricked or misled into voting for someone who has "no chance at the office" and therefore "taking away votes from the Democrats". It is even more insulting to say "a vote for (Nader/other 3rd party candidate) is a vote for (Republican candidate)". The people who vote for the Republicans are the primary ones responsible for the Republicans winning office. That and all kinds of irregularities with ballots, lobbyists, and other things that need to be fixed regardless of what party you're in or which candidate you support.
Edit: For a much more eloquent statement of the above from Nader himself, please see today's Meet the Press transcript.
I voted for Nader in 2000 and 2004 and do not regret it at all. I would not have voted for the Democratic candidate in either of these elections had Nader not been running. I voted for Nader instead of the Green Party nominee in 2004 because I didn't feel that nominee was a good choice for the office. In the primary this year I voted for Cynthia McKinney because I felt she was a good candidate for the office and, unlike Nader, was already on the Green Party ballot. But if Nader is on the ballot in the general election rather than McKinney I will support him. (As you might guess if you know me well, race and gender play absolutely no part in my decisionmaking here, but that's a topic for another post.)
I take my votes seriously and refuse to settle for "the lesser of two evils". Nor do I consider the Democrats and the Republicans to be fundamentally the same, however. Essentially, a vote from me means that I have confidence in that candidate to reflect my values. The Green Party is the only political party that comes close to reflecting my values and I will continue to support them for that reason. If everyone says it's just not a good time to support Greens or other third parties right now, then we will never go beyond the status quo. Enough votes for these parties would qualify them for federal matching funds, and enable them to have a real voice in the election process. Therefore I do not consider these votes wasted.
I will continue to vote my conscience. I respect everyone's decisions and rights to vote for whoever they choose, but I do not respect the opinion that Nader or any other third party or independent candidate is a spoiler who shouldn't run. Don't vote for him if you feel that way, but don't tell me that those of us who do are hurting your party's chances.