As one looks at the electoral map and figures out how Obama (or McCain...) can get to 270 electoral votes, the state that seems to be the most important -- the tipping point -- is Colorado. Of course, there is still a lot of time between now and election day, and if one candidate opens up a large lead in the polls, then no singular state will be terribly important. Put another way, if Obama wins Ohio and Virginia and Florida, he'll have won the election with over 300 EVs, so it is hard to classify any particular state as crucial. But if the race stays close, if national polls continue to put Obama and McCain within 2 points of each other, then I believe that Colorado holds the key to the election.
First, why Colorado? Well, more than any other state -- including Florida, Ohio, New Mexico, Nevada, Pennsylvania & Michigan -- the polls in Colorado have consistently bounced between McCain and Obama, with neither of them ever establishing a consistent lead. Sure, other states are close, such as the ones mentioned above, but poll after poll tends to show the same guy winning. Poll after poll from Pennsylvania and Michigan, for example, show Obama with small leads -- but they always show Obama with a small lead. I liken this to tossing a coin ten time and it landing heads 10 times in a row. Sure, there's a chance this is due to random chance (1 in 1024, for those keeping score at home) but much more likely, Obama has a small but consistent lead in those states. Ditto for McCain in Florida -- never a large lead, but the majority of polls show him ahead.
Colorado is worth 9 electoral votes, and if Obama has the Kerry states plus Iowa and New Mexico, he's at 264, it would put him over the top.
Colorado has been trending blue for the past few election cycles. The governor (Ritter) and both state houses are Democrat. One Senate seat is up for grabs this election, as Wayne Allard (R) is retiring, and the current polls show Mark Udall (D) beating out Bob Schaffer. The other seat is occupied by Ken Salazar (D), who beat out Pete Coors (yes, that Coors) in 2004, a year that otherwise gave the Democrats very little to celebrate. Colorado has 7 House seats (which you already knew, because it has 9 electoral votes). 4 are currently occupied by Democrats, including the aforementioned Mark Udall as well as Ken Salazar's brother, John. The notable Republican of the remaining troika is Tom Tancredo, who was one of the (many) candidates for president this primary season, largely running on a platform of blaming immigrants for everything.
Interestingly, the Salazar brothers are Hispanic, and Spanish is the primary language in ~10% of the Colorado homes, according to the 2000 Census. Colorado's population has gone up 13% since 2000, disproportionately to non-whites, mostly Hispanics, but including increases in the black and Asian populations as well. In general, Obama has been doing very well with Hispanic voters (ballpark of 2-1 Obama-McCain).
In terms of voting, Colorado is one of the states that allows early voting, meaning that you can wander down to some locations and cast your vote early (as opposed to the slightly-more-onerous process of requesting an absentee ballot, although you can do that too). In 2006 Colorado experienced a nightmare with electronic voting machines (also known as DREs, for Direct Electronic Recording), as it was discovered that simply placing a magnet near the machine would cause it to go inactive. I just visited the Colorado Secretary of State's website to find out how they're voting this November. According to a document that had been revised on September 12, 2008, 6 of the 64 counties will use only electronic voting machines (I have yet to figure out which brand) while another 27 will have both paper voting and DREs -- it is not clear if polling places will offer both, or if it will vary by polling place. Anyway, just pointing out that if Colorado really is the tipping point state, well, there won't be any hanging chads... Oh, I should also note that Diebold has apparently changed their name to, or at least sometimes operates under the pseudonym of, Premier. Kinda reminds me of RJ Reynolds Tobacco changing their name to the Altria Group.