There are cables available (about $10 in Canada) that do this. They've been available for 5 years or more. Is this the kind of thing that you mean? :?:
Use CP2101 from silab,it has a Uart and a USB port.It provides all the signalling including DTR,DSR,RI,DCD,Rx,Tx.....etc.CAn be used for transrecieving.....from USB to RS232 and rs232 to USB.
We have used FTDIchip.com products in small projects. FT245 was the type, if I remember right. You can download free drivers and it is easy to communicate to from any MCU/MPU or FPGA/CPLD. These chips also have a parallel interface, if you are not married to the idea of using 232.
connect usb signal to usb transceiver which converts +/-D signals(differential logic signals ) to normal signals( one signal) !!
connect that signal to rs-232 !
a usb transceiver will be key try for different transceivers in fairchild, TI,philips etc !!