I'm more with Andie on this one, although I think Wojtek raises interesting questions.
Marx's typology still holds up well, IMO; those who own the means of production, distribution, and exchange and do not sell their labour power to earn a living are, strictly speaking, "capitalists".
Small producers like farmers and small distributors like corner grocers are simply small capitalists.
I don't have much problem identifying them as "petty bourgeois" since I don't believe that ownership in itself confers power. But, then again, neither did Marx. He viewed this intermediate social layer as a declining one, being squeezed out historically as capitalist development polarized society into two contending classes - consolidating big capitalists, and the wage-earning class, whose power was potential rather than actual.
Today, there are many fewer small farmers and shopkeepers and self-employed professionals in the developed capitalist countries, as Marx forecast, although some feel they have been replaced by a new and rapidly growing "petty bourgeoisie", mainly located in the vast bureaucracies of modern private and public corporations.
I agree the class position of supervisory, professional, and administrative employees has become blurred - their conditions are far different than those of the 19th century industrial proletariat - but they are still employees for all that, the "new working class." They're not exempted because their standard of living has become quite the opposite of what Marx anticipated, although that has certainly prevented them from developing the consciousness and behaviour of a "class-for-itself".
Wojtek's definition of "effective control" (usually over hiring and firing) is often used as a criterion to exclude supervisors from the main body of workers organized in trade unions, but in most advanced capitalist countries, with the typical exception of the US, they are still allowed to form their own unions and to bargain on their own. They are hardly distinguishable from other workers in terms of their wage dependency, status, culture, and living conditions.
The class position of the higher-paid administrators and professionals is still more difficult to identify. However, I can't agree with Wojtek's proposition that if you occupy a position to "the degree that such control affects the functioning of social, economic and political institutions" that you are by definition a member of the ruling class. The state and other corporate boards of management are instruments of ruling class control; they serve the ruling class, but are not always part of it. Labour and social democratic parties, for example, have governed on behalf of capital even though they have originated in, and for the most part, are still based in the working class.
As to the individuals who occupy these higher-level corporate or government offices, my view is that their class position is, again, based on how they primarily earn their income. Most, as we know, live an affluent lifestyle derived from a mix of salary and other income from profits, interest or rent. I would say that if it is the latter which determines their living standards, and their salaries and bonuses are supplementary, then they can be considered as part of the capitalist ruling class. If they're mainly dependent on their salaries to maintain their living standards, however, I would be hesitant to include them. They could better be described as the most lordly of the "labour aristocracy".
Class location, of course, is of more than academic interest. I think it's still true, even in this period of limited social strife and class consciousness, that how individuals earn their living is a key determinant of how they behave politically. Those who are dependent on wages and benefits to maintain their living standards will typically support liberal parties whose tax, spending, and other legislative policies strengthen their capacity to secure them. Those whose income derives from profits or the ownership of other assets typically favour conservative parties which call for lower taxation, less spending on social programs, and more stringent control over the poor, the powerless, and dissenters.
This division is admittedly less clear in the US, where a larger share of the working class votes against its economic class interests because of the social and national prejudices which tend to be prevalent in the population of large empires. But even here, I think it's clear that the mass of urban workers - blue collar, service, technical, administrative, and professional - supports the Democratic party, and that the Republican base is located more in rural states and rural counties where the social weight of small propertyholders is greater.