I had a look at a similar AVR project (*) a few days before. They used an Attiny 2313 for less than 2 Euro to implement an USB <=> SPI master. IIRC to clone a standard ISP. That's cheaper than an FT2232 and even available in DIP package for the breadboard fans and home brewer.
This solution has some limitations, of course. <half knowledge>e.g. it doesn't support bulk transfer and therefore cannot replace a real USB <=> UART bridge.</half knowledge>
@lillitroll: Funny that we had the same idea at the same time. I thought about it, but I think it's only possible if one reads the USB standard regarding the correct timing. The author of the Attiny stuff wrote that his implementation is still according to the standard, but pushes it to the limit.
However, it can't be too complex, since the AVR implemetation fits into 2 kByte of flash
(*)
http://www.xs4all.nl/~dicks/avr/usbtiny/
Thomas
(Interesting voltage regulator in the schematic above, D1 + D2 :lol: )