I bought a 8GB SD card for my Raspberry Pi and happily dd’ed the Raspbian image to it. But it did not boot. @namp and @eliaschr suggested that I re-dd-ed (redid?) the image to the disk. But I know how to run dd(1), right?

Yes and no :(

I run dd.exe from a windows machine:

C:\TEMP> dd.exe if=2013-09-25-wheezy-raspbian.img of=\\.\e:

Ding! The above does not start writing from the beginning of the disk. It starts writing from the beginning of the E:\ partition. I dd-ed the image from a Mac and all boot well.

I suppose (but have not yet checked) that had I used the \\.\path\to\physical\drive path, I would not have bumped into this.