Since Oracle doesn't provide the file deb for Ubuntu, you need to download Oracle SQL Developer for other platforms (usually is a zip file that doesn't not include a JDK), and follow the instructions below:
http://oracolodeidb.blogspot.com/2009/04/ubuntu-e-oracle-sql-developer.html
PS. As described in the article, I also needed to execute the following command:
$ cd ~/.sqldeveloper
$ echo /usr/lib/jvm/java-6-openjdk > jdk
