OK, other people have also written about this, but the information seems to be cluttered all over the place in different forums or the CentOS wiki.
The most straightforward way I found to do this is as follows. Credit for this method goes to Neil Aggarwal, who posted it on the centos mailing list.
In case that link stops working someday, it is as simple as:
1. Using fdisk, I created two partitions on the drive:
/dev/sdc1 = 15 MB
/dev/sdc2 = The rest
I marked the first one bootable
2. I used dd to copy the diskboot.img to /dev/sdc1
3. Formatted /dev/sdc2 as ext3 and copied the
iso images there
4. When I boot from the usb drive, I selected hard
drive install and pointed the installer to
It worked perfectly!