Here's what I sent back.
He's scanned all the responses he received and uploaded them here. He's had an absolutely amazing response. There are letters from Wim Crouwel, Daniel Eatock, The Sagmeister, Tom Geismar, Milton Glaser, Wally Olins, Aziz Cami from The Partners, Alistair Sim from LOVE, Simon From Poke, Lewis Moberly, Alan Dye from NB:Studio, Phil Carter from Carter Wong Tomlin, Harry Pearce from Pentagram and many, many more. Have a look at them all here.
I know I'm biased, but I think our letterhead looks bloody brilliant amongst that lot.