If you want to participate in the 2008 Presidential caucus process in Colorado, you must be registered to vote and affiliated with the political party of your choice by December 5, 2007.
Unaffiliated voters and people who register to vote later don't have a say in who the Democratic and Republican Party nominees will be in 2008. Colorado has a bigger say this year than in any, probably in the last hundred years, because the earlier February 5, 2007 allows us to have a say before the other states have already made the decision for us.
For prospective candidates, the word is tough luck. It is to late to affiliate with a political party to run as a candidate of that party in 2008, or to register to vote at all so that you can run in non-partisan races (like the RTD Board) in the 2008 election. But, there's always 2009 and beyond. If you are already registered to vote in the party of your choice, however, the clock is ticking. Most candidates for state and federal office will be nominated through the caucus process to appear on the Colorado primary election ballot in March. You can petition onto the primary ballot as well, but few people do so successfully.