If you want a through-hole USB-Serial chip, the only one I could come up with when I looked in the past is this Microcontroller:
http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en533924
Another option is the PIC32MX250F128B
You can buy it with preinstalled USB-bootloader for $ 3.75
http://www.microchipdirect.com/ProductSearch.aspx?Keywords=TCHIP-USB-MX250F128B
It is very easy to get going, just plug the USB-cable to the chip (almost XD )
http://chipkit.net/chipkit-bb32/
And you can use MPIDE to program it, very Arduino-like
MPIDE is a fork of the Arduino IDE
I have put it on a stripboard, the black wire on the left is the USB-cable
