It's possible, but it gets more and more unlikely every day. Obama's got solid leads in enough states to give him the electoral votes he needs. McCain's way far behind, according to every metric I've seen. There simply aren't enough toss-up states (with enough electoral votes) left for him to make up his vote deficit. Here are some pretty decent electoral vote tracking/simulation sites:

www.fivethirtyeight.com
www.270towin.com
www.pollster.com
www.realclearpolitics.com

Don't believe what the mainstream media is projecting. They're cherry-picking polls to find the closest margins between the candidates, hoping to perpetuate the notion that the difference is razor thin.