The most general definition of a patriot is one who loves and defends his country. Conflict arises continually over how a would-be patriot interprets that definition and puts it into practice. Certain interpretations definitely fall outside the realm of right and reason. Asabiyyah is one of the greater sins akin to prejudice and includes patriotism or nationalism in at least some of their manifestations. For example, when one turns a blind-eye to injustice and wrong committed by a government because of one's origins or nationality, that is a great sin. Similarly, seeking to defend the indefensible or serving colonial and nationalist aggressions in any arena is also wrong.
In reality, most or all national governments have both good and bad qualities and commit both good and bad acts, and at times have both good and bad intentions. A thinking person must be able to accommodate these complexities into his worldview, and thus there can be no true thinker who is a blind nationalist or patriot. Nor can there be a true thinker who is wholly against every single aspect of a particular nation or group of nations, either. McCarthyist tactics, such as the recent firing of Octavia Nasr from CNN for tweeting that she "respected" the recently departed Sayyid Muhammad Hussain Fadlullah, exists in every extremist worldview. They are among the ugliest interpretations of patriotism and cannot be supported whether for or against one's nation or "side".
As for the aspect of sports and celebrations, a believer should ponder the intentions and the images portrayed in what he or she does. There are no rulings against displaying a flag or marking most national holidays that exist in the various countries, barring any prohibited acts. But the word "fan" is short for "fanatic", an extremist position that is both rationally unsupportable and unseemly of a believer. If we take the Ahlul Bayt (peace be upon them) as our role models, it becomes immediately clear that even if World Cup Fever is "harmless halal fun", it may not really be entirely harmless in its extremes and is at best a pointless amusement. Even in our imaginations we realize that it is not befitting of our role models to don the colors of some nation, whether it is a Muslim-majority country or not, and wildly cheer on its team in some sport. We know the Ahlul Bayt would not do this, and we know our religious leaders do not do this. This fanaticism wrongly promotes athletes and entertainers as the elite heroes of our youth over those who live Islamic ideals and exhibit quality character in difficult circumstances. While sports and celebrations can be healthy, character-building and family-building, and can also promote Islamic values, those benefits are only present if they are consciously cultivated while the negative aspects are actively removed.
As for matters such as a pledge of allegiance, national anthems, and civic oaths, whatever oaths a Muslim takes, whether explicit or implied, are binding on him, but nothing supersedes the Straight Path. One might take a principled stand against reciting the Pledge of Allegiance or not, depending on one's interpretation of certain lines. Whatever stand one takes, it should be an informed, intentional, and meaningful stand. But regardless of that, the duty to neighbors, regardless of their beliefs, is an Islamic one that could fall under certain interpretations of patriotism. A Muslim should serve and honor his country by living as a Muslim who honors his/her duty to neighbors. Muslims should seek to serve their countries by helping them to evolve in positive directions, by standing up against oppression, by helping the needy, and by dealing ethically and morally with all peoples in all circumstances. It does not serve or honor one's country nor Islam to support, participate, or excuse the wrong acts of one's government or people.
So can one be a Muslim and also a patriot? It comes down to interpretation. If loving and defending one's country means supporting and overlooking its wrongs and defending its sins and aiding its aggressions, then absolutely not. Neither can blind fanaticism for a nation, even just for sport and leisure, find any support in the Islamic canon. But if loving and defending one's country means supporting its good qualities or ideals and striving to improve its negative ones, aiding one's neighbors, and actively serving in various positive forums in one's communities, then this kind of patriotism is warranted and, in at least some aspects, required of all of us.