Regular contributor Anton Philidor keeps asking who owns Solaris - implying that an open source product is community owned and therefore ultimately nobody's responsibility.
Technically, of course, Sun has ceded control of the openSolaris code base to a community in which it is the major player, while retaining all rights and responsibilities for the core Solaris OS for itself -meaning that Anton's wrong to begin with.
Realistically, however, he has a good question: Solaris really is free, so how does pouring money into advancing Solaris make sense for Sun?
There are several compelling answers to that - starting with the reality that Unix has always been a free, as in beer, product that some people more or less successfully charged for. Thus AT&T at times wanted $75,000 for a source tape with a highly restrictive license, but the Berkeley Systems Distributions were widely available in academia for the cost of the media - just like the original Bell Labs releases and today's Solaris 10.
Interestingly, the people who have tried to charge the most for Unix -IBM, DEC, Red Hat, Microsoft, and, in some periods, AT&T - have also generally done the least with it. Convesely, the commercial players who muttered about licensing to satisfy their obligations to AT&T but actually gave it away with their hardware - Sun, HP, NCR, Honeywell/Bull - have done the most.
Basically people who don't understand that Unix should be free, don't understand Unix - and by forcing the application of inapplicable ideas cause their own failure.
Commercially, however, this is what MBAs and journalists like to call a "business model" issue - meaning that how you make money from a free product depends on what you really charge for. Thus Red Hat sells free linux licences in the same way that Microsoft sells free Windows upgrades, while Sun actually does give away Solaris licenses but then offers various levels of add-on support - the key difference being that Sun is honest about the real deal while the other two are not.
What's going on here is that giving away the product - whether openly the way Sun is doing it or by strategically encouraging illegal copying the way Microsoft does it - opens new markets for paid licenses or services. That's critical, for example, for Sun's embedded java marketplace where Java's value (suitability to the purpose) combined with near zero cost for vendors lets them achieve unit volumes in the hundreds of millions - which then drives demand for products and services both from Sun and from the huge number of smaller businesses comprising the third party Java ecosystem.
In effect, therefore, OpenSolaris represents the direct continuation of a strategy that has proven itself since Ken Thompson first sent a Unix tape to a colleague at Berkeley: giving away the product builds volume and customer commitment, encourages the adoption of new ideas, "surfaces" bad ones, and builds community by building markets for secondary products or services.
But all of it, of course, depends on continuing product innovation: a process that's made more responsive by open source contributions, but which nevertheless requires Sun to keep throwing expertise at it. So, what's the bottom line answer to Anton's question? Sun's continuing investment in Solaris is about building short term volume by providing long term value.