Ubuntu 9.04 Jaunty Jackalope has been released earlier today and a lot of people have actually installed it on their workstations, like me!
Right after installing, I tried updating my repositories (list of packages, correct me if I used the wrong word). I’ve noticed that it stopped updating itself at one point.
You may experience this problem when clicking the Reload button on the Synaptics Package Manager or when shooting the sudo apt-get update command on the terminal.
Here’s a workaround to get it done. Now, all you have to do is to go to System -> Administrative -> Software Sources. On the Ubuntu Software Tab (the default), you will see a download from dropdown box. If you are downloading from the Main Server or your country’s server, try choosing other choices on the Other.
Try reloading the repositories again (or shoot of the sudo apt-get update command. That should work.
Too many people are updating their repositories right now, that’s why Main Server or your country’s local server is not cooperating that well. 
If it won’t, then you’ve got to do more Google. Hahaha.