To export a path from AI (CS5) do the following:
See also http://creativedroplets.com/transparent-png-with-illustrator/
- Select the path(s) you want to export
- Copy and paste it somewhere if necessary
- Draw an artboard somewhere with the artboard tool
- In the artboard presets select fit to selected art
- Now export as png and check the box ‘use artboards’
- Select your artboard
- Select the correct resolution (if the resolution is different from the AI source, the png size will change accordingly)