The ATtiny series do not have USB capability, which is why they are cheaper. The underlying bitbang implementation comes from V-USB [1], which is commonly used to add USB capability in software. Another implementation which may be of interest is LUFA [2].

[1] http://www.obdev.at/products/vusb/index.html [2] http://www.fourwalledcubicle.com/LUFA.php