I recently had a lot of troubles in making my Gentoo communicate with USB->serial converter, which was working on a prolific 2303 driver.
It turned out, that when driver is compiled as a module and is not hard compiled into kernel, it just works, otherwise it doesn't.
In a concrete way, what I could see in the logs was, that when driver was compiled into kernel, the device was recognized (dmesg showed, that new device was attached and also could recognize type of device (prolific device), but the line, that device is attached to ttyUSB0 was missing), when the driver was compiled as a module, device was properly recognized and properly attached to ttyUSB0.
My question is does udev make some assumptions, that some drivers are available only as modules? What could have happened?