It's frustrating that the Lightroom beta only lets you work with one library, and that by default it's created in My Pictures. A quick workaround is to close Lightroom and cut and paste the library from My Pictures to wherever you want. Next time you open the program, it'll ask you where to find the library.

That solves the matter of where to store the library, but some of us “need” to have more than one library for different projects. Well, move your existing library as above, restart Lightroom and instead of finding the library's location, select create a new library and immediately close Lightroom. This creates a new blank library in the default location, so simply cut and paste it to where you want.

You can repeat this process to create multiple libraries. Double click the aglib file to force Lightroom to open it, or hold down the Alt key immediately after you double click the Lightroom icon - this launches the File Open dialog.