Øyvind:
I really suggest if your timeframe is so short that you do not have time to "design the hardware" that you most likely also do not have the time to dig into all the software aspects of "messing with USB software".

I would follow the solution that Erik has shown you of using a USB to RS232 bridge type product. The Cygnal CO2101 is one fine choice for this. If you look hard at this you will see that it is a good way to go because of the ultimate simplicity of the development process that this brings. On the product end you simply pick out your boards or processors with UART ports there and simply make the embedded application as if it was a serial I/O solution to the PC. On the PC side you simply install the VCP driver (== Virtual Communications Port driver) and the application there interfaces to a Windows COMMx: port number.

For a quick low cost way to get going with the USB end of things you do not even have to go the route of buying the Cygnal CP2101 evaluation board. You could actually buy a number of these cables....(See this current eBay auction for "really inexpensive") and buty the thing inside your enclosure.

Thus for prototyping and possibly even initial production of product you can select almost ANY 8051 board that is easy for you to program and has the I/O you need and the UART Rs-232 port to which you can attach the above shown type of cable. (Goodness you could even buy Cygnal processor evaluation boards if you wanted to). To satisfy the USB pass through in the inital products you can then simply get some small hubs (such as these types as seen on this current eBay auction). You could even rip the plastc off the hub if you wanted!!

Later as the product takes off and you have some breathing space with the schedule then you take on the idea to lay out a board of your own. On this board you put the type of 8051 that you selected, a Cygnal CP2101 chip for the USB serial bridge (Note that this chip is setup to attach right to the logic level UART pins of your microcontroller without a RS232 level converter in between. On this same circuit board you can bundle up a USB hub too. That is almost as simple as a single chip and some USB connectors. Several companies make these chips and there is no new software to write to get it going. One such chip is available from Philips. Here is another example of TI's chips for this function....with even the RS232 bridge function built right in.

Good Luck
Michael Karas