You really need to do a "Making of" video for that sort of thing. It's nice having both the e-presence and the physical Christmas card to send (or to have already sent out) to clients.
An agency my partner worked for did "Jumpers for Shelter" last year: employees hand-knitted or hand-accessorized a jumper, which was then auctioned along with some bling donated by clients (kitchen knives, computer games, jewellery). All money from the eBay auction went to Shelter.
I suppose if he were to sell the PDF, it would automatically turn his "encourag[ing] redistribut[ing] the PDF, including via BitTorrent" into piracy. If you see piracy as a necessary evil, you might grumble at that; if, like Minecraft's creator, you see piracy as a nullity:
and say "there's no such thing as a lost sale", then you might not care in the slightest. But the nature of the ecosystem surrounding your book will inevitably change: as someone said elsewhere in this thread, you can't loan an eBook; a solution to that would be sending someone the PDF, but you've just turned them into a pirate.
There's no easy solution. Upselling blogposts might be a better idea; even then, it would be good to have figures.
An even more radical alternative would have been Mark Pilgrim's approach to Dive Into Python. You could give your book a Creative Commons licence and expect people to copy and remix it:
Content Delivery Networks offer good scaling opportunities. In my limited experience, pricing of bandwidth and storage space scales better with S3 than most shared hosting alternatives. Amazon's CDN also takes care of the actual resource scaling for you, whereas shared hosting would max out.
With S3: if nobody turns up, you just pay storage for the PDF (less than pennies, $.14 per GB); if everybody turns up, Amazon handle the extra load and you pay a proportional amount in bandwidth fees.
With shared hosting: if nobody turns up, your hosting fee remains the same size (pounds or dollars); if everybody turns up, the shared host falls over and the hosting company invokes some clause in their contract with you to force you to host with S3 instead.
I think the big problem is that plaster walls just aren't flat enough to prevent ink hiding in the divots and creases. So you can wipe over the surface but never get the little spots out of those imperfections.
We've painted a whiteboard-sized area of our office with this stuff. There's no border to tell you where the whiteboard paint finishes. As you'd expect, the ink is no longer confined to the bit that's wipe-clean. Otherwise (with the spray proviso above) it's been very useful, and we can always extend it with another pot later.
Another alternative is those statically charged pieces of "whiteboard paper" - Whiteyboard is mentioned below, and there's also http://www.magicwhiteboard.co.uk/ - although they don't stay up tidily for much longer than a couple of weeks. I think they're only really meant to last e.g. a day-long meeting.