Actually really annoying, I had at least 5 a few months ago but now I can't find a single bloody one, I keep handing them out to others with on it and I either don't get them back or they must be falling through holes in my pockets!
I was in need of my Arch Linux boot USB the other day as well so I could diagnose another computer with a boot from USB option, but alas it was nowhere to be found. I haven't used CDs in years now because USB booting is far, far handier than burning a new CD / DVD every time you want to try a new distro out. It's just a pity that not that not every distro distribute USB images.
Did I hear rumors of Windows avaliable as a USB image in future? That would save on CDs as well, although I don't like the idea of 4+GiB for an OS install, even if the latest USB drives are bigger than that now.