Recently upgraded a site from 6.0 to 6.1.2... now when I hit Host > Extensions, I have this error at the top of the page:

 The following packages are invalid:
E:\inetpub\vhosts\mydomain.com\httpdocs\Install\Provider\WURFLClientCapabilityProvider_06.01.02_Install.zip

At this point I'm unable to add new extensions (nothing happens when I hover over Manage > Install Extension Wizard and click).

Most other functions on the site seem fine. Any ideas?

This package should not have remained in the Install\Provider\ folder following a successful install. Apparently when building the grid of available extensions this package's manifest file cannot be properly parsed resulting in the error you are observing.

Did you notice any errors during the upgrade process - please check Event Viewer, the 06.01.02.log.resources file in the Providers/DataProviders/SqlDataProvider/ folder and the log4net log files located in the Portals/_default/Logs/ folder for any related errors that might have occurred during installation.

Next, let's delete the offending package from the Install/Provider folder. You should be able to open Host --> Extensions normally.

I would then look through the installed extensions to see if the "DotNetNuke WURFL Client Capability Provider" shows up in the providers section as having been installed.

If it was not installed successfully, download a fresh copy of the DotNetNuke 6.1.2 upgrade package from http://dotnetnuke.codeplex.com , unblock the downloaded package and extract it into a temporaryy folder. Then, using the Install Wizard from the Host -->Extensions page, try installing the WURFLClientCapabilityProvider_06.01.02_Install.zip that you'll find in the Install/Provider folder of the extracted DotNetNuke 6.1.2 upgrade package. Be alert for any errors noted during install.