Google CEO Eric Schmidt has resigned from his post on the board of directors of Apple, which he has held since 2006. His resignation reflects the degree to which the two companies, once comfortable bed-fellows, are now in competition with one another.

"Unfortunately, as Google enters more of Apple’s core businesses, with Android and now Chrome OS, Eric’s effectiveness as an Apple board member will be significantly diminished, since he will have to recuse himself from even larger portions of our meetings due to potential conflicts of interest," said Apple CEO Steve Jobs in a statement. "Therefore, we have mutually decided that now is the right time for Eric to resign his position on Apple’s Board.”

The growing rift between the two companies was evident last week when Apple disallowed applications based on Google Voice, the search giant’s telephony technology, from the iPhone App Store.

Furthermore, Google’s recent announcement that it is developing a desktop operating system, the above-mentioned Chrome OS, adds yet another battle-front between the two companies, already competing in the mobile sphere.

With mobile devices becoming an increasingly critical component of business infrastructure, and the gradual decay of Microsoft’s desktop hegemony, the next operating system war that CIOs watch with interest may be waged between two giants of consumer IT.