Colorado is a Obama-leaning tossup (Obama +0.6 -not what you'd call a "blue state"), and, according to RCP, the "Others" option is polling at 4.2 so there you have Johnson.
And all the current polls already factor in the probability of the person showing up to vote- that's why in "registered voters" polls, Obama has a +4/+5 extra lead compared to the "likely voters" polls. Which he is also leading. Romney needs all the polls in the country to have consistently underestimated him (or overestimated Obama) to win.
"Other" is not the same as naming the third party candidates. Because these candidates will appear on the ballot, they should be included in polls. Gary Johnson has been as high as 13% in his home state when he is named.
The Green Party also has a candidate who is on the ballot in enough states to theoretically win the election, as does the Constitution Party.
Also, more people say that they're going to vote third party than actually do. They end up feeling that its not worth waiting in line to cast a purely symbolic vote.