SP2101 is too expensive solution for my purposes. I found uP that are several times cheaper then Silicon labs one and has USB and there are lot of free resources to put my code there - see HOLTEK 82K96.
And yes, I agree that Silicon labs USB - serial converters are really excellent... just too expensive and there is no place to put my code. I used uP 8051F320 from Silicon labs and it has plenty memory after you add the USB kernel, unfortunately it's 3 times more expensive then HOLTEK.
you know, this USB thing JUST seems to be too complex, once you've sent the host the necessary descriptor blocks, it will provide you with a device class driver, so for many standard applications, like modems, audio devices, video capture and mass storage, you do not NEED to develop any kind of PC-side driver...
I think that for low data-rates (a maximum of 64kbytes/sec, unidirectional) a simple CDC enumeration would do the job...