I am having the same issue as posted here Prolific PL2303 and Ubuntu 12.04 (only in english). The usb to serial adapter worked fine on 11.10 but after upgrade to 12.04 it is not recognized in lusb.
if i connect the device after boot and then run: dmesg | grep tty
[ 101.305275] usb 2-1.4.4: pl2303 converter now attached to ttyUSB3
seems like it is working fine, but a running the command few seconds later adds the line:
[ 107.788672] pl2303 ttyUSB3: pl2303 converter now disconnected from ttyUSB3
after this the device is no longer detected at all.