The “plus” in Microsoft’s Software+Services strategy is going to get a lot more oomph in the not-too-distant future — at least according to a planning memo for Windows Live Wave 3 that I had a chance to see recently.

The memo — authored by Chris Jones, Corporate Vice President of Windows Live Experience, David Treadwell, Corporate Vice President of Live Platform Services, and Brian Arbogast, Corporate Vice President of Mobile Services — dates back to the summer of 2007. But it’s all about the future of Windows Live — and how Microsoft’s family of Windows-complementary services are going to get a lot more cozy with Windows and Internet Explorer.

Recent leaks regarding Microsoft’s next-generation Windows 7 release have all mentioned that the next version of Microsoft’s client operating system will be more tightly tied with Windows Live. The degree to which the two will be joined at the hip will become clearer, as Microsoft marches towards delivering Windows 7 in late 2009 or 2010 and Windows Live “Wave 3″ at the very end of 2008.

Even though the Windows and Windows Live engineering teams both report to Senior Vice President Steven Sinofsky, the two product families have had relatively integration and cross-pollination to date. But with the next releases of these products, a lot is going to change.

Some choice excerpts from the memo:

“We will invest to deliver a seamless experience for customers who own a Windows PC. We have a unique opportunity to remove the seams between Windows, our applications, and our services. Windows Live Wave 3 will be designed so it feels like a natural extension of the Windows experience.

“While we will target a seamless experience on Windows Vista, we will make a bet on the Windows 7 platform and experience, and create the best experience when connected with Windows 7. We will work with the Windows 7 team and be a first and best developer of solutions on the Windows 7 platform. Our experiences will be designed so when they are connected to Windows 7 they seamlessly extend the Windows experience, and we will work to follow the Windows 7 style guidelines for applications.”

And more from the memo:

“Windows Live Wave 3 will be designed so it feels like a natural extension of the Windows experience. … We will ‘light up’ the Windows experience with Windows Live. … What’s the relationship between a Windows account and a Windows Live ID (Microsoft’s Web-authentication technology)? Should we have a LiveID connected to account settings?”

The bottom line, according to the planning memo: “Windows Live will have value for every Windows customer. If you have an email account and use the Internet, Windows Live will make your experience better.”

It sounds like Microsoft’s current strategy of making Windows Live Photo Gallery an extension to Windows Vista is a very tiny tip of the iceberg. One of Microsoft’s main challenges will be to avoid being torpedoed by new antitrust lawsuits by its competitors and the European Union as it pushes to get the two product families in sync.

What kinds of Windows+Windows Live scenarios do you hope — and fear — Microsoft will deliver,going forward?