Assuming that one has begun a PhD program at a university which allows a student to choose an advisor after his/her first year in the program and the student has narrowed down to a couple of potential advisors, what is the etiquette for approaching them for advisor commitment? Specifically,
Advisors understand that students are fishing around - that's normal. What's less common (but possible) is students fishing around in wildly different areas. Just keep in mind that either an advisor is going to see something in you that will motivate them to try and convince you to work with them, or they'll view you as someone who needs to make up their mind first, and won't spend too much time thinking about you.
In that respect, matching up with an advisor is more like dating than an job interview, at least in the US :)
Ben Norris is basically correct—as advisors, we know that students have multiple options for choosing advisors, particularly in setups where the students pick advisors after their arrival. Therefore, we won't (or at least shouldn't) take it personally when someone tells us they're considering other advisors.
In general, however, you should also remember that these processes are often double-sided: in case of competition, the advisors also have a choice in who they want as their top choice. If you are overly hesitant, the advisor may choose another candidate who is more certain as a top choice. So, you should be honest if you're not immediately "sold" on working for a particular advisor, but don't be so negative as to make the advisor look elsewhere.
The adviser already knows that you are not 100% committed to his or her research group. If the system lets students choose, then the advisers know the rules. The faculty in that department very likely made those rules. They know that you are considering other advisers.
It is never rude to consider other advisers. Some programs even require it. My graduate program required that I interview at least three potential advisers and rank them. The faculty would then be informed of the list of students who chose each faculty member at the first choice only. Faculty had to fill open positions from this first.
This system was implemented by the faculty a few years previous, when they became dissatisfied with an approach that left matching entirely up to students and faculty meeting and getting along. Faculty would then choose their favorites from all interested students. Usually, this meant that a small number of students were not chosen at all, which was not acceptable.
