According to dictionary.com:
Pa"tri*ot*ism\, n. [Cf. F. patriotisme.] Love of country; devotion to the welfare of one's country; the virtues and actions of a patriot; the passion which inspires one to serve one's country. --Berkley.
na·tion·al·ism Audio Help /ˈnæʃənlˌɪzəm, ˈnæʃnəˌlɪz-/ Pronunciation Key - Show Spelled Pronunciation[nash-uh-nl-iz-uhm, nash-nuh-liz-] Pronunciation Key - Show IPA Pronunciation
–noun 1. national spirit or aspirations.
2. devotion and loyalty to one's own nation; patriotism.
3. excessive patriotism; chauvinism.
4. the desire for national advancement or independence.
5. the policy or doctrine of asserting the interests of one's own nation, viewed as separate from the interests of other nations or the common interests of all nations.
6. an idiom or trait peculiar to a nation.
7. a movement, as in the arts, based upon the folk idioms, history, aspirations, etc., of a nation.
The #2 definition in nationalism is interesting because it refers to patriotism while #3 talks about an excessive patriotism.
Anyway, I believe they are different. I often joke when someone wears a flag pin or a flag on their car or whatever that they are obviously true patriots because they are flying the flag. I also have a red shirt that happens to have a flag sewn into the collar that I call my 'Gay Hating, Bible Beating, Beer Loving, Gun Toting Patriot Shirt' that I wear to special occasions (or when it is the only clean shirt left).
See, I believe that there is a certain strain of what people call patriotism that runs through this country that is actually nationalism in its ugly form. The type that says 'America can do no wrong' and if you dare raise a question or dissent in anyway you are labeled a traitor. It was used as a club during the 2002 and 2004 elections and is still used to demonize people that question our policies on torture and wireless wiretapping, etc. And it is all simplistic bullshit. Wearing a flag pin and yelling 'America is great and the French suck' isn't being a patriot, that is being a 5 year old.
One of the descriptions of the difference between patriotism and nationalism that I like is that nationalism is country above all, no matter what. Patriotism is the love and defense of the values the idea of a country hold above all. So when you start to piss all over the Constitution if you aren't questioning it you are a traitor.
So yea, in my view there is a stark difference between the two. Unquestioning loyalty or informed dissent. I'll take the latter, thank you very much.