In reply to Ali:
Having read a lot about this, here is the deal. DWRs are like lots of little spines sticking up out of the coat. You can't feel them, but they're there. Abrasion / rain / grease / sweat all make these spines lie down, and when they lie down water doesn't bead off them.
What makes them stand up again?
1) A lack of contaminants
2) Heat
You will notice a significant improvement in the DWR of any coat by tumble drying, even the nikwax coated ones that claim not to be "heat activated".
This is from an email convo I had with a mammut tech rep. I think similar advice can be found on the rei.com website.
So wash all of your outdoors stuff in techwash or pure soap (NEVER DETERGENTS - they strip the dwr, NEVER FABRIC SOFTENER - they force the spines to glom together and down), then take them to a local laundrette and run them in the medium/cool cycle until toasty.