When I made my first business trip to the Middle East in the mid-70s, I was told to cover myself head-to-toe. No need for an abaya, chador, cloak or hijab, but I should wear a full skirt that covered my knees (mid-calf or full-length would be even better) and my blouse should cover my elbow and fasten at the neck. If I chose to wear trousers, although not recommended, it was to be very loose.
With the obvious exception of Saudi Arabia, this conventional wisdom has remained the norm. Yet, even before the Arab Spring of 2011, the dress code for women had begun to change in several countries in the region.
Dressing modestly remains the goal. Form-fitting trousers, see-through blouses or tight T-shirts remain culturally insensitive. Most Middle Easterners make inferences about someone's status by their clothing; dressing well denotes status in Middle Eastern cultures.
That the world becomes smaller every day is borne out by the changes in what women may successfully wear in the Middle East. Trousers have become increasingly fashionable in many countries as long as they aren't form-fitting. They make it easier to sit without worrying about how much leg is exposed. In the office, trousers should cover the ankle.
If you're traveling to Abu Dhabi, Dubai and Bahrain, your skirt can skim the top of your knee. However, and this is important, your skirt must be the same length when you sit down. So, pencil skirts are out, but full skirts and dirndl style skirts can often do the job.
In the same region, women can wear sleeveless blouses as long as they aren't tight and the fabric isn't revealing. The neckline must be modest (absolutely no cleavage).
Like most rules, there are exceptions. If your company policy allows clothing that may not be acceptable outside the office, it's a good idea to keep a scarf with you, or carry a pashmina in case you have to meet with someone who is ultra conservative. These items can come in handy when you're not at work and you don't know the religious inclination of people on the street. The pashmina can also be a handy cover for your shoulders when the air-conditioning is set too high.
When visiting Iran and Saudi Arabia, women must cover their clothing with either an abaya or manteau and wear a headscarf. In Iran, the hijab should not allow any hair to show (for tips on how to accomplish this, check out the Role of Women article in the Iran Country Profile on CultureWizard). In Saudi Arabia, women should wear an abaya and headscarf. Outside Riyadh, the headscarf may often be omitted, although regulations vary by region and change frequently and without warning.