Nationality, race and religion have always been interesting, relationship-wise. For some people it's an obstacle and for others it actually can be something that draws you closer to eachother. When looking at nationality it usually doesn't matter except when there is some history between the two countries. For example: A Chinese girl/guy dating a Japanese girl/guy.
Religion is even stricter about this thing. I still have to meet the first christian-muslim couple. On the other hand, my father was catholic and my mother was protestant, so it's not impossible.
Race used to be quite an important factor and it probably still is for conservative people. The funny thing is, that there's some double standard w.r.t. race and relationships. For example, if a black guy dates a white girl it's considered to be "not normal" by some people. On the other hand, when a white guy dates a black girl, those very same people consider it to be okay. I think this is due to the old role patterns of men and women. The man "captures" the woman, so in effect when a black man dates a white girl, the white people "lose" and when a white man dates a black girl, the white people "win".
Just an explanation of what some people think, they are not MY ideas. I think a relationship shouldn't have anything to do with religion, race, culture and/or nationality. In fact, it's quite odd that humans are one species but still manage to seperate people into groups. Blacks, whites, asians etc. are for like 99.9% similar and just because 0.1% (the skin color) is different, we immediately start to seperate. I think it usually comes from a lack of social contact between different races. As soon as you how similar you are, you can accept that skin color is something totally irrelevant.
Just a little rant. ;)