Yep, you'd need to disconnect the USB before hooking the PS3 to the modem's ethernet port.
See
http://homepage.ntlworld.com/robin.d...b.html#usbswap
If you have an ethernet port on your PC, you'd be better off using that instead of USB (& if you don't have one, ethernet cards are very cheap). And, if you used ethernet for the PC, you could then get a router, & remove the need to swap leads & reboot the modem.