I believe the World file option is only available if you export from a data frame, not the page layout.
From Exporting your Map
The TIFF file format has the additional ability to store georeferencing information internally. This is a GeoTIFF. To create a GeoTIFF, click the Save as type drop-down arrow and click TIFF, click the Options arrow to expand the options, then click the Format tab and check Write GeoTIFF Tags. This option is only available when you export while in data view.
While this information relates to exporting from the UI, the same logic probably applies to ArcPy.
I regret I can't test it right now.
I just tested it, and the world file option works with a data frame, while the "PAGE_LAYOUT" yields an error. This makes sense because the page layout is in page coordinates i.e. (0,0) to (page_width, page_height) not real world coordinates.
EDIT#2: Warning, this tool does not work as you may expect. (I'm using Arc10sp1). If the specified image size does not have the same aspect ratio as the data frame, unseen portion of the data frame are exported to fill the image. However, the x and y pixel scaling in the world file is based on the extents of the data frame shown, not as expanded for the image.
If you use the UI tool (File->Export Map...), you will notice that the image size is preset to the aspect ratio of the data frame and you cannot change the image size except by changing the image resolution.
A work around is to get the extents of the data frame, and then calculate the image aspect ratio to match. Like so:
mxd = arcpy.mapping.MapDocument("CURRENT")
df = arcpy.mapping.ListDataFrames(mxd)
ar = df.extent.height / df.extent.width