A major departure at Microsoft (NSDQ: MSFT) this evening: Kevin Johnson, president of Microsoft’s Platforms and Services Division, is leaving the company to head up gearmaker Juniper Networks. The departure will coincide with a reorg that will see Johnson’s unit split into two parts. Microsoft’s official announcement is here. Added: Juniper’s announcement is now out here. The news was first reported at WSJ.

Johnson, who has been with Microsoft since 1992, and at his current position since 2005, was seen as a major figure in the company’s attempt to acquire Yahoo (NSDQ: YHOO). In his official bio, the company touts 30 acquisitions under his watch in support of its ‘software+services’ strategy.

As for the reorg of his unit, here’s how it works: One unit will oversee the Windows operating system and related online services. The other will oversee the pure online-only aspects, like MSN.com and the online advertising business. The first group will be headed by CEO Steve Ballmer, and three other executives. The company will begin a search for a newly created role to head up the other unit.

See more here from Kara at AllThingsD, who has also obtained a copy of a memo sent out by Steve Ballmer. It covers a wide range of stuff that must keep Ballmer up at night: Here are some key parts:

-- On Competition With Apple: “In the competition between PCs and Macs, we outsell Apple (NSDQ: AAPL) 30-to-1. But there is no doubt that Apple is thriving. Why? Because they are good at providing an experience that is narrow but complete, while our commitment to choice often comes with some compromises to the end-to-end experience. Today, we’re changing the way we work with hardware vendors to ensure that we can provide complete experiences with absolutely no compromises. “

-- On Competition With Google: “In the coming years, we’ll make progress against Google (NSDQ: GOOG) in search first by upping the ante in R&D through organic innovation and strategic acquisitions. Second, we will out-innovate Google in key areas—we’re already seeing this in our maps and news search. Third, we are going to reinvent the search category through user experience and business model innovation. We’ll introduce new approaches that move beyond a white page with 10 blue links to provide customers with a customized view of their world.” (Ed note: they’ve been saying this for a long time, especially that ‘10 blue links’ part)

-- On Yahoo deal: “Related to Google and our search strategy are the discussions we had with Yahoo. I want to emphasize the point I’ve been making all along—Yahoo was a tactic, not a strategy. We want to accelerate our share of search queries and create a bigger pool of advertisers, and Yahoo would have helped us get there faster. But we will get there with or without Yahoo.”

-- On the Reorg: Actually, this is the one part where there’s not much that wasn’t in the press release.

• Kevin Johnson: Yahoo Offer Fair; Business As Usual; Thinking About Brands; The Culture Thing