Conducting Personal Business on company Time: How ethical is that? And where do you draw the line?

Amany: Thanks for starting this thought-provoking thread!
I already see many great answers here.
Adding my two cent's worth:
1. Personal work on company time is almost completely unavoidable, especially in unavoidable situations where the appointment being made cannot be done outside office hours. Joshua Render has mentioned a few specific situations.
2. As Kiron Bondale rightly pointed out, even the unavoidable may have to be stricly avoided if the contract mentions it. For example, in highly controlled environment where client confidentiality is de rigeur, even connecting to the internet is forbidden, and personal phones are not even allowed in the workplace.
3. Sante and Dinah pertinently show that, in a majority of situations, official work is inevitable in personal time. In some cases, such work eats up a lot of personal time, causing serious work-life balance issues.
4. In my personal experience, most modern superiors are not bothered about short unavoidable breaks as long as the assigned work gets done well, and in time.
5. However, it is completely unethical to use workplace resources for one's own business needs.

All of us have to make a judgement call on how much personal work is too much. And that decision can be tricky.

In the end, we have to adapt to specific workplace situations and contract terms, never letting the "personal" work affect the client's prorities.

It also depends on what you are considering "personal business". I think everyone here has address it as things like doctor appointments and other miscellaneous personal tasks. That is very different than if you are running a home revenue generating business on company time. That is generally looked at very differently than the aforementioned reasonable personal use of company resources and people get fired for that.

I concur with my colleagues here. It depends on the organization and culture. However, I think there are certain things you cannot do at work and maintain your professional reputation.