If you’re one of the many looking to download a new version of Ubuntu the moment it’s released (Natty Narwhal at the time of writing), you’ll receive the “Could not download release notes” error from the Ubuntu Update Manager if you’re using the main Ubuntu servers (or even your country’s main server).
This can easily be fixed by selecting an alternative server source (permanently or temporarily – it’s up to you if you want to change the source back again later). Click ‘Settings’ at the bottom left of the Update Manager, then the ‘Ubuntu Software’ tab. The ‘Download from:’ option area will list the main server and the main server for your country, but if you click ‘Other…’ you should be presented with a greater list of options. Try one of them (even one in a nearby country if you’re short on alternatives), and you should find you can start your upgrade to the latest Ubuntu version successfully.
Hopefully in future versions update-manager will have a slightly more informative error message, or maybe even suggest automatically switching to a different server if failures or slow speeds are experienced. I’m surprised this isn’t already encouraged considering the benefits of load balancing.