When I originally installed Ubuntu on my DecTOP, I didn’t have any problem booting from a USB flash drive or an external CD drive. Reinstalling Debian on the same box, however, was not quite so easy.
For my own future reference, plus anyone else who may run into the problem, the key is to hold down ESC while the DecTOP POSTs to get it to boot from an external USB drive (at least for a CD drive, presumably a flash drive as well). Shortly you should see a CD-ROM check box appear next to the memory test box and you can let go.
Update 10/13/10: I just went through the install process once again and still had a lot of trouble getting it to boot to the external CD drive. The key is actually to hold down ESC then press power. The DecTOP passes the post tests too quickly for you to hit it after turning the machine on and seeing the display wake up.