WASHINGTON (Reuters) - When Democrats gather in Denver next week to nominate Barack Obama for president, they'll be joined by such uninvited guests as Republicans Rudy Giuliani and Mitt Romney. The two former presidential candidates will be among two dozen or so Republicans in the city hoping to get their party's message out during a week dominated by Democratic festivities. Their slogan? "Not Ready '08: A Mile High and an Inch Deep," a play on the nickname for the high-altitude city in the western United States. < > They will be joined by Minnesota Gov. Tim Pawlenty and former Hewlett-Packard...