First of all, for any software or hardware platforms, you want to make sure the product you select implements open standards interfaces, so that you are not locked into using only one product. This gives you the flexibility to move from one product to another if you run into security, scaling, support, or cost issues.
However, if the product is open source, because you can see how the interfaces are implemented, it makes reverse engineering them a much more simple process. Being able to see the source code also aids with interoperability, because it removes any ambiguity on how the interfaces are implemented.
The other big advantage to selecting open source is that you can get support from multiple vendors. For example, with Solaris you can get support from HP, IBM, DELL, Sun, INTEL, Fujitsu and AMD (also true for Linux). So anyone can provide support because all of the Solaris code is in the public domain. This also gives you, what I like to call "investment protection." If a company provides you with an open source product, and then goes out of business tomorrow, the code is still in the public domain, so you can easily get another company to pick up your support requirements.
This also saves money because there is competition in providing the support. For a proprietary product, only the company that owns the code can support it. With open source, since you are not locked in, you can compete the support from multiple vendors.
That's a HUGE deal, because in the government sometimes the cycles are so long that a selected vendor could go out of business or completely change its product direction during the life cycle of your project, and that could force a very expensive change and/or extended time line. Also, vendors can often only provide support for one version of their product for about 10 years, and many government projects live longer than that, so by selecting open source products, it gives the government many other support options, for example a Systems Integrator could pick up support for an open source product version that has been EOLed by a vendor.
The selection of an open source product keeps you from being locked into a vendor and provides "investment protection" throughout the entire life of your project and beyond the life of the vendor and that's really, really important in this wild unruly world of mergers and acquisitions, changing economies and those kinds of crazy things 
