UPDATE: OS restrictions were removed from Lightroom 3.5 license, see comment #4 below. Windows support is now back.
Previously, I’ve been under impression that my Lightroom 3 license covers both OS X and Windows platforms. I originally got that idea after reading the Lightroom FAQ, which is a bit vague about the platform issue.
Subject to the terms of the software license agreement, the primary user of the computer on which Adobe Photoshop Lightroom 3 is installed may install a second copy of the software for his or her exclusive use on either a portable computer or a computer located at his or her home, provided that the software on the second computer is not used at the same time as the software on the primary computer. Lightroom is sold as multiplatform software, which means it can be installed on either Mac OS X or Windows.
Apparently, this is not the case, as the official license says it more clearly.
2.10 Dual Boot Platform. The Software is licensed for use on a specific operating system platform. You must purchase a separate license for use of the Software on each operating system platform. By way of example, if you desire to install the Software on both the Mac OS and Windows operating system platforms on a device that runs both of those platforms (i.e., a dual boot machine), then you must first obtain two separate licenses for the Software. This is true even if two versions of the Software, each designed for a different operating system platform, are delivered to you on the same media.
As a result of this new information, PLUGINS POSTED ON THIS SITE ARE NOT SUPPORTED ON WINDOWS. Effective immediately, June 7th 2011. From this date onwards, the plugins are only tested on OS X.