Give me a break.
The average American, if an Arab doesn't cooperate by wearing a full beard and a dishtowel and riding a camel waving a Koran(and I suspect that Al Quaeda, if it does use Arabs again...and they've stopped using them completely for ANYTHING in the US, by all appearances...will not have their operatives thus attired) is pretty much totally incapable of accurately determining who is "Middle Eastern" or "Arab" or not.
The reality is that as an ethnic group Arabs are a highly varied community that has had an enormous amount of intermixing with neighboring populations (and those populations, like the Spanish, who have then intermixed with Native American populations in the Western Hemisphere, and black Africans, have intermixed with Arabs themselves.)
All kinds of Hispanics, Italians, Greeks, etc. could easily be mistaken for Arabs, and plenty of Arabs look like African Americans, or don't look "Arab" at all; cleaned up and wearing a suit, a fair number of the 9/11 hijackers don't look particularly Middle Eastern at all.
And then, of course, Pakistanis and such are not Arab, nor are they "Middle Easterners."
Read on one the the threads an article that states on Padilla's marriage license it shows him to be an African American using the name Ibraham.