Heh. My all time favourite thank you note is from my BIL & SIL. When they married, we gave them a toaster, brand spanking new, wrapped up in its box and we told them that if they didn't like it we still had the receipt, so it could be exchanged or returned. They assured us that it was exactly what they wanted.
Fast forward to after their honeymoon, when they decided to unpack their brand new toaster and have toast for breakfast. Out came the toaster, in went the plug, in went the bread, down went the lever... and there was an almighty BANG and a fountain of sparks, as the toaster was faulty!
The thank you note:
"Dear parrot_girl & King Parrot,
thank you very much for attending our wedding. Thank you also for the lovely toaster; there would have been easier ways to kill us, but none quite so cunning. Love from BIL & SIL. P.S. Can we get the receipt off you?"
I will treasure it forever.