Manchester encode a data stream
GolfScript 28 characters
{2{base}:|~4|43691-~256|~\}%

Equivalent version without obfuscating optimization:
{2base 4base 43691-~256base~\}%

The code accept input as an array of integers, and return ditto.
For each number in the array the number is converted to base 2 array form, it is then converted back to a number as if it was base 4, this has the effect of spacing out the bits with a 0 in between each. 43691 is then subtracted from the number, and the result is binary inverted, this is equivalent to subtracting the number from 43690 (43690 = 0b1010101010101010). The number is then split into two parts by converting it to a base 256 array, the array is decomposed and the order of the two resulting numbers is inverted.
Example input:
[1 2 3 241 242 243]

Example output:
[169 170 166 170 165 170 169 85 166 85 165 85]