Myself, I'd love to see the following (some of which have already been suggested, admittedly);
Iron-on embroidered patches of the logo as used at the top of the website.
T-shirts, especially long-sleeved ones (I love those, and they're not offered as a promotional item nearly as often as I'd like).
Lanyards are always useful!
SD Cards (both blank and with pre-installed OSes for the Pi) and USB flash-drives are a must. (Bonus points if the flash-drives are those silicon-cased ones and are shaped like the logo.
Hats, such as beanies or baseball caps - or both. Propellor beanie optional.
Raspberry Pi-branded keyboards and mice. It would be especially nice if a compact "laptop-style" keyboard was an option, in addition to a full-size/full-width one (am I the only one who far-and-away prefers the compact type?). It would be even cooler if these had Raspberry Pi logos screen-printed on the super-key, but just including a sticker for it instead would be just fine!
Raspberry Pi-branded USB hubs of various types would be very good to have, too. I'd definitely pick up a powered ten-port one if it were available with the branding, myself.
I also really like the previously-suggested idea of a Tux beanie plush with the Raspberry Pi logo embroidered on his belly, a shirt, or wherever.
That's all I can think of off of the top of my head. I would definitely buy all of these.