Google has made more than 4,000 Android Wi-Fi hotspots free across the U.S. in collaboration with Boingo Wireless. There is a bit of a catch, though: if you use a Windows Phone, iOS or Blackberry device, you might not be able to connect.

These Android Wi-Fi hotspots will be located in areas that see a lot of traffic like malls, cafes and hotels, and users will be able to connect with laptops running Windows, OSX, Linux or Google’s Chrome OS in addition to Android devices. 15 airports in cities such as New York, Chicago and Seattle support these hotspots, as well as subway stations in Manhattan.

Boingo recently acquired marketing firm Cloud Nine Media, and their technology helps to support these free Wi-Fi hotspots. When users connect to the network, they will be taken to a landing page encouraging them to browse Google Play content. According to Boingo’s Dave Callahan, “sponsorships like this give users the free Wi-Fi they crave, advertisers the consumer interaction they need, and venues the revenue to offset the costs associated with providing a high-bandwidth Wi-Fi experience.”

Because no Windows Phone, iOS or Blackberry devices can use content from Google Play, this leaves them out of the offer, as there is nothing in it for Google. Whether this means that users of these devices will need to pay for access or simply won’t be able to connect is unclear at this time. Since there is no way of knowing whether laptop users own an Android device or not, they are still able to connect.

Instead of checking MAC addresses, Boingo is checking devices by their browsers’ user agent data. This means that users of other devices may be able to get around the restrictions by using a tool that allows them to customize their user agent string such as the Atomic Web browser for iOS or User Agent Switcher for Windows Phone 7.

Have you seen these free Android Wi-Fi hotspots in your city? Do you mind advertising if it gets you free web access?