This one could potentially be much more damaging for BlackBerry than any of the other shifts so far. The US General Services Administration is an independent government agency, and one of its main tasks is to provide supplies and communications for other government agencies. In total, the GSA manages $500 billion in government assets, including quite a bit of telecom and IT. As such, the GSA not only has a direct impact on what devices other agencies use, but the policies that the GSA enacts tend to become blueprints for other agencies.
The trouble for BlackBerry is twofold here, because not only is the GSA planning to issue iPhones and Android devices to 17,000 of its workers, but it is also in the process of testing the use of employee's personal phones and tablets on government networks. This would be a way to cut costs for the government by not needing to supply employees with secondary devices, but it also means that BlackBerry will be losing more and more traction in a market that it has traditionally dominated.
Right now, BlackBerry accounts for about 95% of GSA devices, so that could be quite a hit for RIM. The GSA has said that it doesn't have any plans to stop using RIM servers though.