Grimlock, last week I contacted/chatted with Pentax USA with that exact question. At least from this service rep's response, it looks like it is fine to purchase the extended warranty from any place, so long as you keep a receipt and purchase it within 90 days of purchase:
Pentax Imaging Chat of February 2, 2011
Please wait while we find an agent to assist you...
You have been connected to Matthew P.
Matthew P: Thank you for contacting Pentax Chat Support. How may I help you?
Dan: Hi. I bought a K5 recently and was interested in the 2-year Pentax extended warranty. I purchased it through Camera Land in New York and right now they do not have any warranties to give. Can I purchase it through another retailer instead? For example, B&H?
Matthew P: You will have to search their website for K5 2 year extended warranty
Matthew P: We can always help you if you're within the first 90 days of having the camera.
Dan: I only got it a couple of weeks ago. My question is whether I can purchase the warranty from a different retailer than where I purchased the camera.
Dan: You have them for $100, right, or is the price different? Thanks.
Matthew P: Yes. You can.
Matthew P: 99.95.
Matthew P: It's within the first 90 days though.
The agent is sending you to
Digital Cameras and Accessories - Official PENTAX Imaging Web Site.
Matthew P: I suggest checking out 'Warranty Informationa.'
Matthew P: Information*
Dan: So is it true that I cannot purchase the extended warranty from a different retailer than where I purchased the camera? Thanks.
Matthew P: You can purchase an extneded warranty from a different reatiler from your original purchase.
Dan: Okay, thanks. So there is no need to register the warranty, just to keep the receipt of purchase. True?
Matthew P: You do not need to register your camera to keep the warranty.
Matthew P: Just keep the receipt.
Dan: Okay, thanks for your help, Matthew. Have a good day.
Matthew P: You too! Thank you for using Pentax.