Had a quick look at doing this, the only way to organise things sensibly would be to allow voting on Blueprints and bugs (as opposed to allowing users to entering any old rubbish). At least this way some degree of quality would be ensured, along with some hope of developer support.
Unfortunately I fell at the first hurdle: Launchpad doesn't support RSS feeds. These would be needed to import items into the Ideabuntu site. Otherwise it wouldn't be difficult to get a prototype working using Drupal and it's Vote Up/Down module.
If anyone knows better than I and Launchpad actually supports feeds, then please provide a link. Otherwise I'll raise a feature request (am not holding out much hope though).