While they are considered,and, if you go into the data files and change Politicalparty to 2, you can be one, you cant have a realistic presidential race without them. I just used this program at a model United Nations at Yale where we had a Third Party candidate running against us and winning 38 electoral votes. You cannot do that yet in this game, so when and if Political machine 2008 in available, i think third party candidates is a must.

Considering the last time a 3rd party candidate got any electoral votes was 1968, I think any sim that would have them win 38 is way off base. In '68 Wallace took 46 electoral votes, all in the south. He won 12% overall but I don't think he got hardly any votes outsed the south, if he was even on the ballot elsewhere. It was a symbolic run because he could not possibly win the election.
In 2000 Nader was a spoiler by getting 2.7% of the total vote, but no electoral votes. He made the difference in states such as Florida and made the race close in Iowa and Wisconsin despite finishing less than 5% in all of those states. Third party candidates can only be a spoiler. Even Perot didn't win any electoral votes in '92 when he took almost 19% of the popular vote.
So tell me again why third party candidates should be considered? They are already considered as the dissatisfied voters who go to neither candidate. There is no reason to go beyond that.