Are PhD scholarships and assistantships taxable?
Taxation does depend very much on the country, as well as the type of award you receive. The amounts similarly vary from nation to nation. To give two examples:


In the US, scholarships and fellowship stipends are taxed as ordinary income. However, in some cases, for externally funded awards, you need to be careful, as the university might not withhold taxes. In that case, however, you would need to make estimated tax payments, as you are still responsible for paying the tax "on time!" 
Don't forget that your tax burden may also include state and local income taxes, depending on where you live. Making matters even more complicated, cost of living fluctuates wildly: you're probably better off with a $25,000 award in the midwest than a $35,000 or $40,000 award in New York City.
In other countries, the system can vary. In Germany, for instance, graduate fellowships are not taxed, while "standard" graduate positions, which are considered employees of the state, are taxed. At the same time, however, people receiving the taxed positions receive health benefits and pay into the social security system. Stipend recipients are responsible for their own health insurance, and do not accrue time in the social security system.