When I bought my Dell 530n with Linux a few months ago, I also went out and picked up a Belkin Flip that allows me toggle the keyboard/mouse/video between the Dell and one of my other PCs. Been generally happy with it except for one problem - the Dell hangs on boot if plugged into the Flip. Every time I rebooted, I would unplug the flip and plug in the keyboard directly. Kind of annoying since in the last few weeks I have been doing Linux kernel development which requires frequent reboots.
Finally got around to making a bootable USB thumb drive (since the PC doesn't have a floppy drive) and updated to the latest BIOS (1.0.10). Problem all fixed. PC now boots as expected when using the Flip.
On a sidenote, Linux makes it WAY too hard to make a DOS boot disk, even though FreeDOS is readily available under the GPL. I don't need DOS very often (pretty much just for BIOS upgrading), but still with Windows it's as simple as checking the "make DOS boot disk" box on the format dialog.