Cicso's "sweet spot" is harnessing the network. However, remaining a pure network player stifles Cicso's growth. Their foray into unified communications (UC) laid the groundwork for owning two layers of the infrastructure -- the IP network, upon which the email network resides. Cicso's acquisition of IronPort bolsters this two-layer approach. Separately, the acquisition is further evidence to the splintering of the email security space.
In November 2006, IronPort (email and Web filtering) announced their acquisition of PostX (messaging encryption), establishing a strong security base for text-based messaging. (The acquisition is due to become final in late February 2007.) Cisco's UC solution is voice-centric. However, UC encompasses voice and text, and I expect that Cisco will re-engineer IronPort/PostX technology for securing their UC. A broadening of their UC solution into the text space also seems to be likely.
Meanwhile, Microsoft is posing an early threat to both the UC and messaging security spaces. They refocused Exchange 2007 from an email/collaboration product to the "cornerstone" of their new UC platform, with a heavy investment in voice -- and their new Forefront platform securing it all.
I expect an interesting onslaught of bedfellows.