Long a partner to the big server makers, the networking equipment giant today became a competitor, announcing an aggressive push into the server market. No longer content to peddle switches and routers alone, Cisco (CSCO) is now selling what it calls a unified computing system–a full-blown data center solution that encompasses everything from servers and storage to connectivity and virtualization services. The move is a brazen challenge to IBM (IBM), HP (HPQ) and other vendor partners with whom Cisco had once cooperated. “We’re going to compete with HP,” Cisco CTO Padmasree Warrior told The Wall Street Journal. “I don’t want to sugarcoat that. There is bound to be change in the landscape of who you compete with and who you partner with.”

Of course, but “change” is rather a tame word for a potentially market-disrupting expansion of Cisco’s business. This is a power grab, plain and simple. A game-changer. Cisco is offering an integrated approach to what’s long been a multivendor arrangement. Whereas before, CIOs would purchase servers from one company, virtualization software from another and networking from yet another, the networking giant is proposing they now purchase them together from a single vendor: Cisco. And that puts it on a collision course with IBM and HP.

“H-P, IBM and Cisco are the new four horsemen of IT infrastructure and they are all fighting to increase their share of the enterprise IT wallet,” Forrester Research analyst James Staten told Marketwatch. “They have all benefited from growth of the market and by taking share from weaker players, but are now needing to go after each other’s strongholds to keep growing. They are definitely leveraging technology evolutions that drive unification, so customers win through this competition, but it’s going to be a bloody fight.”

Uh-huh. Well, it certainly wasn’t Cisco’s share price, which has fallen more than 20% from its November high …