There are two problems with using USB sensors; short cable lengths and an insufficient number of USB ports. To solve this, Inside Out Networks sells their AnywhereUSB/5, which is a five-port USB hub that connects using Ethernet. It is one of the few USB over IP boxes on the market today.
You can connect up to five USB devices to the AnywhereUSB/5 and then put it far away from your monitoring computer. In addition, you can have multiple AnywhereUSB/5 hubs on your network.
Before you can use the AnywhereUSB/5, you need to install a driver that tricks your computer into thinking the remote ports are local on your PC. Then, after connecting the hub to your network, you open up the AnywhereUSB Concentrator Configuration Utility. This will automatically detect the box and help you set an IP address for it.
The AnywhereUSB/5 works fairly well. We popped a 512 MB Sandisk Keychain into one of the ports, and it showed up just as if it had been inserted directly into our laptop.
You don't have to buy the AnywhereUSB/5 to use the environmental sensors, but it makes life easier, because you have more freedom in placing your sensors. The AnywhereUSB/5 sells for around $399.