Sun is making some progress in the x86 market, particularly at the high end. IBM has opted to further adoption of Solaris OS by making it available on more of their high-end System x servers and BladeCenter servers, both of which have gained in popularity the past few years.

Previously, IBM hasn't offered or considered Solaris, which may have limited their potential in certain enterprise markets that relied on it. Sun benefits in two ways from this deal, not only from more exposure and sales for Solaris but with increased partnership with IBM:

By adding the Solaris OS to its operating system portfolio, IBM is expanding customer choice. And by participating as one of over 800 partners in the IBM BladeCenter ecosystem, Sun has effectively joined others in the industry helping IBM accelerate the development and adoption of open blade server platforms.
You can read the full press release on IBM's site.