Just for your information:
I tried to crack the access code of the Yubikey using a C program based on the ykpersonalize project. Turns out it takes a pretty long time to crack the 48-bit code: With my program I get around 13 codes per second, at this rate (If I'm really calculating it the right way.) it would take sever thousand years to crack the code.
If you want to try it out yourself, you can find the code http://stacksmashing.net/2010/04/19/yubicrack-brute-forcing-a-yubikey/