The two standards (USB 1.x and 2.0) have entirely different frame timing and signalling requirements. USB 2.0 devices are backwards-compatible, but USB is a bus, and all of the devices on a bus segment have to use the same frame timing.
On a homogenous USB bus (all 2.0 devices, say), the addition of a new (idle) device should not affect the bandwidth of any other devices. The USB standard allows bulk packets (the kind used by mass storage devices) to use up all of the remaining time in a frame, after higher-priority packets like isochronous and control are sent. If the other devices are idle, they should not affect bandwidth at all.
Newer versions of Windows actually alert you when a 2.0 hub has reverted to 1.x mode, so you know you're going to lose performance. Some computers have only one internal hub which services all of the ports, meaning that attaching a single 1.x device to the computer will slow down every other 2.0 device attached to any other port. (Mice, keyboards, and other low-speed interface devices are generally 1.x.) Some computers, however, have multiple independent hubs.
Hubs can "promote" 1.x devices to 2.0 devices, as well. In other words, imagine that you attach a USB 2.0 hub to a port on your USB 2.0 computer, and then attach all your 1.x devices to that hub. The link between the hub and all the devices will be 1.x, but the link between the computer and hub will still be 2.0. Sometimes you can use this to your advantage by "quarantining" all of your 1.x devices to a single hub, so your computer's internal hub(s) can keep operating at 2.0.
- Warren