Arabs are Caucasians since they have the same origins as Europeans. It's also important to note that Caucasians evolved in the Middle East (near the Caucasus, west Arab Levant ,north of Iran).
The ''Caucasians'' include: Arabs, North African (Berbers), Iranians, north Indian Subcontinental people, Europeans etc. The predominant racial type in all those areas is Caucasian. Even though all these Caucasian groups have some non-Caucasian admixture to some degree (including Europeans) they are still more genetically closer to each other than to non-Caucasians.
For instance, brown-skinned Arabs are more genetically similar to blonde North Europeans than the Japanese are to Malaysians, and in fact, the racial divide between the North Asians (North Japanese, Korean, North Chinese) and Southern Asians (such as Malaysian, Thai, Filipino) is much much deeper than the one between Arabs and Europeans.
In forensic science and anthropology, ''Caucasian'' is used to describe a skeleton/skull type, rather than a skin color. You can have pale-skinned Caucasians and dark-skinned Caucasians. The skin color is the least important factor since skin color correlates with lattitude and doesn't necessarily indicate a populations true racial origins.
Arabs are Caucasian not because of mixture with Europeans, but because they have similar origins as Europeans. There is a big difference there.
http://en.wikipedia.org/wiki/Caucasian_race
http://weirdmusic.net/images/caucasian_race.pdf
http://en.wikipedia.org/wiki/Arabid_race
