How do I update my package cache using Ubuntu 12.04?
Ubuntu maintains a cache of packages that are used when installing a new product. If a recommended package is not in this list, you will be unable to install it. In this case, you will need to refresh the cache before installing the recommended package:
If you have the RTM version of Ubuntu 12.04 and have never installed any of the Ubuntu updates, Steam will fail to install. Applying these updates will fix this.
It is important that your package cache is up to date when you launch Steam (or when you select Steam | Check for Video Driver Updates ...) because Steam compares your current video driver against the current contents of your package cache.