From http://effbot.org/zone/element-xpath.htm:

[position]	(New in 1.3) Selects all elements that are located at the given position. The position can be either an integer (1 is the first position), the expression “last()” (for the last position), or a position relative to last() (e.g. “last()-1” for the second to last position). This predicate must be preceeded by a tag name.

Testing shows, that [0] is accepted, and seems to be [last()]. 
However [-1] selects no elements. I think these expressions should raise an exception. (Or the feature should be 0-based and behave like python list indices)