IGN is reporting that the PC version of Borderlands has been pushed back six days in North America and ten days internationally for further "optimization". The console versions are apparently unaffected:
The PC version is sliding back from its original release date of October 20 in North America to October 26. The international version of the PC game will slide back further to October 30. The North American PS3 and 360 versions of the game will be unaffected, and the international versions for the PS3 and 360 will still ship on October 23.
We contacted 2K Games for an explanation on the slight delay for the game, and Charlie Sinhaseni, Senior PR Manager said, "We're optimizing the PC version which takes a few days longer than expected. Borderlands for PC will ship on October 26th."