This is just a heads up for anyone having trouble with some themes. At the moment, the WordPress.org theme server seems to be having some serious issues. It is randomly delivering incomplete and incorrect .zip files of some themes. So far, 2010 Weaver and Piano Black are known to be affected.
To test this, simply go to the WordPress.org page for either of those themes, and repeatedly download the .zip file. At this moment, Piano Black 2.2 is being downloaded as a 299KB and a 147K .zip file. 2010 Weaver is downloaded as a 4K, 496K, and 1.5MB .zip file. (the 1.5MB is correct).
If you try to upgrade, preview, or install one of the broken .zip files, you will get errors or white screens, and may have to use your host cPanel (or equivalent) or ftp to delete the theme directory.