GolfScript, 13 chars

~2\?,{.2/^}%`

For example, the input 3 produces the output [0 1 3 2 6 7 5 4].

Here's a de-golfed version with comments:

~              # evaluate the input, turning it from a string into a number
2 \ ?          # raise 2 to the power given by the input...
,              # ...and turn it into a list containing the numbers from 0 to 2^n-1
{ . 2 / ^ } %  # xor each number in the list with itself divided by 2
`              # un-eval the list into a string for output

GolfScript, 13 chars

~2\?,{.2/^}%`

For example, the input 3 produces the output [0 1 3 2 6 7 5 4].

Here's a de-golfed version with comments:

~              # evaluate the input, turning it from a string into a number
2 \ ?          # raise 2 to the power given by the input...
,              # ...and turn it into a list containing the numbers from 0 to 2^n-1
{ . 2 / ^ } %  # xor each number in the list with itself divided by 2
`              # un-eval the list into a string for output

It's perhaps interesting to note that there are no particular "golfing tricks" involved — this is basically the most obvious and straightforward way to solve this task in GolfScript.

GolfScript, 13 chars

~2\?,{.2/^}%`

GolfScript, 13 chars

~2\?,{.2/^}%`

For example, the input 3 produces the output [0 1 3 2 6 7 5 4].

Here's a de-golfed version with comments:

~              # evaluate the input, turning it from a string into a number
2 \ ?          # raise 2 to the power given by the input...
,              # ...and turn it into a list containing the numbers from 0 to 2^n-1
{ . 2 / ^ } %  # xor each number in the list with itself divided by 2
`              # un-eval the list into a string for output