Apparently this is where you re-order your PDF pages so that when you print them out two per page and double sided it forms a cute little booklet.
You need "psutils" installed.
psbook -s44 inputfile.ps outputfile.ps
where the -s tag sets the number of pages your document needs. This must be divisible by 4. So, for a 43 page document, you need -s44, and the extra blank page will be added. Neat.
Then you can do the following to go back to a pdf:
ps2pdf outputfile.ps newoutputfile.pdf
When you print, you must select "duplex: short side" or else it comes out all screwy with the reverse pages rotated through 180 degrees.