This acquisition will place Google in the top-tier of spam filtering, and furthers their efforts to become a complete desktop solution from the top down:
Google is framing the acquisition as part of their Google Apps strategy, pitching Postini as the perfect addition to their Gmail, Calendar, Talk, Docs & Spreadsheets, and Personal Start Page efforts. Google will tap Postini's cadre of services to add security, archiving and policy enforcement to Google Apps.
The cost of acquisition will run Google about $625 million, though clearly they aren't shy about investing significant money into new ventures.
However, Postini is a bit different from others they have acquired. Rather than being a startup or a relatively unknown company with a good product, Postini is already known as an “industry-standard” offering, and is definitely not a pay service. What exactly do they plan to do with their new toy?