multiple USB serial adaptors on a SheevaPlug
I just noticed Matthew Palmer's Insane/Brilliant idea of the day: he proposes to use large sets of USB serial adaptors with a sheevaplug as a cheap serial console server.
As part of upstream on cereal, i feel obliged to mention that pacage as a tool for managing serial console farms like this. It's designed to run in a small footprint, stores timestamped logs for the consoles, supports concurrent remote access, and uses standard unix accounts (usually via ssh) to permit read and/or write access to each port. It has saved me exactly the headaches Matt describes many times.
However, i've had trouble getting multiple identical USB serial adapters to persist at standard device file locations across reboot. That is, if you have four pl2303 devices from the same manufacturer, it seems to be a crapshoot which one will be /dev/ttyUSB0 after you restart your system. I could find no distinguishing data in the sysfs to get udev to persistently key off of, anyway. if you know a way to do it, i'd be happy to see it!
Depending on how many ports you need, another alternative would be to use a sheevaplug with a multiport USB-to-serial adapter. While i haven't tried this specific hardware, it would remove the need for the hub, and potentially would mean you didn't need any extra power. I'm assuming that this device would give you persistent port naming, but i haven't tried it. Pricewise, it seems to be a win, too: $100 for the SheevaPlug and $100 for the 8-port adaptor.