I highly recommend you get a copy of Luke Wroblewski's book, Web Form Design. This is my go-to reference manual on thing we take for granted such as labels, location of input controls and action commands.
His topic on labels contains eye tracking tests and case studies of real-world scenarios. These tests and studies have a general suggestion that top-aligned labels be used when you are trying to minimize the time to complete data entry.
In my opinion, the label position depends on the number of "lines" there are in the field:
- If the label is refering to a single line of options then it is more clear to leave it in the same line:
- If the label is refering to several lines of options then it is more clear to leave it on top, as a header occupying its own line:
Also consider consistency within the form and using the same layout for all the labels.