If I want to make an Arduino derivative product based on the Arduino UNO design, and I make no modifications to the firmware loaded on the ATMega8u2 usb chip, do I still have to go off and buy my own VID/PID from USB-IF?
(i) It is generally agreed that you have to buy your own VID/PIC licence at a cost of about $2000 AND they are unlikely to take your $2000 unless you can prove that you are substantially real :-(. See references at end.
(ii) BUT - you appear to be legally allowed to buy VID/PID pairs from legitimate owners under some circumstances. What the conditions are would need to be determined from the licence. See iii
(iii) I am aware that a member of this forum owns a VID/PID licence and has sold VID/PID pairs to enthusiasts. I won't say who they are to prevent possible deluging but will bring this post to their attention. They can speak if they wish.
Gilberti industries discussion October 2010  = No
Official answer here, 6th post down, is not encouraging. October 2010. Essentially - "we are not ALLOWED to give them away but are negotiating to be allowed to do so for use on a non commercial basis" = No in your case. 
Lady Ada wiki. No. 
Things may have changed since late 2010, but it seems unlikely. This is a grave impediment [tm] to any would be roll your own USB designs aimed at small scale production. $2000 is "not a lot of money" once you get into the 1000 - 10,000 production range but below that it starts to be noticeable.