There are conditions that one must meet before being able to participate in the debates. One such rule is that they must have support from a certain percentage of the electorate (I forget if it's 5% or 15%). So, in short, Ralph won't be there.
Wasn't he in one of the 2000 debates? Maybe my mind is slipping.
Anyway, I don't think its more than 5%. I just seem to remember Nader being a bit more involved in the 2000 election.