How do you repair yellowed or faded Lego blocks? In particular, I've had white blocks that have yellowed in places. Can the colour consistency be restored? Or at least improved?

I'd be more interested in people's actual experiences rather than simple suggestions.
When I first got my kids some Lego I did it by bulk buying 25 kilo bags of used bricks off the Internet. Now these bricks had obviously been in outdoor sandpits, chewed by dogs, hidden under carpets - you name it.

We successfully restored almost all of them by by washing them in a bath full of a Milton solution, scrubbing each one with an old toothbrush and then leaving them to dry on a towel. 

This cleared up some yellowing that we had thought was sun damage and fading, however there were some (especially some black pieces) that just looked off-grey and no scrubbing sorted them.

I would suggest trying this method, but if it is sun damage that is more than the outer surface I think you might not have any success.