Well, let's see.... You didn't take the camera SCUBA diving, you were caught out in the rain. You took reasonable steps to dry off and dry out the camera and lens after getting wet, also it looks like you tried to protect the unit on your way back home. Both the camera and lens were sealed, and Pentax advertises Weather Resistant (WR) - who are you going to believe here - Pentax was advertising some level of protection. So, I can see Pentax not covering water damage, if you took it SCUBA diving. But getting wet on the way home, then I have to ask, just what does Pentax mean in their advertising with respect to "Weather Resistant"?
They have pictorial advertising copy out of the camera and body wet (water drops and water beaded up all around it), and with mud all over it sitting on a rock in the middle of a stream. They advertise their 72 camera body seals.
So at best, you should not have to pay full retail replacement cost. They should cover at a minimum 50% of the lens cost (not retail, but what the lens is sold to the retail shop for - wholesale, and no sales or VAT tax either). Given that Pentax wants its users to believe that there is some benefit to WR, then I would say that they should cover 70 to 80% of the replacement cost at wholesale. I know that sealing a camera/lens set is not an absolute science with the variables involved. I know that is not the way they see it, but there is some inherent defect here I would have to think. A 8 month old DA* lens should not be trashed after being out in the rain - based on their own implied advertising. If this is not the situation that WR and the * seals were designed for, please Pentax tell us what the situation is? This begs the question of "how much weather is too much for weather resistance"? If they are going to claim that water damage is not covered, then Pentax should define exactly just what weather resistant actually means in a repeatable engineering sense. How much water - a light fog? a misting of water vapor? a very humid day (less than 30% humidity)? or ??? ? Inquiring minds would like to know.
I would also cite this thread, and the number of hits it is getting. A 2,334 hits currently and counting(I might add), I would say that there is some level of interest in Pentax's response. Certainly counteracting Pentax's advertising to some degree.
Actually, the best thing that they could do, is to quietly replace the lens, take it back to the lab and figure out what went wrong, use it as a learning experience in terms of manufacturing and quality control, improve their processes, and everyone benefits. If, I were Pentax, I would then send it to the lead lens designer for sealing the unit, for his "trophy case" of failures.
This is the level of customer service that destroys a carefully laid out and executed advertising campaign and marketing approach. They put the engineering up front to seal the units, advertised it correctly, and now with a failure - not covering any "water damage", does not make any sense.