Canonical has pushed a security update that will disable all Sun Java plugins in Ubuntu 10.04 LTS, 10.10 and 11.04. Thus the only alternative now will be to use OpenJDK from repositories or manually download Java from Oracle website (or from unofficial PPAs).
There are a number of reasons because of which this step has been taken. Firstly Oracle has withdrawn the permission to distribute Java packages by default in an Operating system and thus Operating System Distributor License is now no longer valid.
Secondly, the current Sun Java version in partner repositories in 10.04, 10.10 and 11.04 has security issues that are being exploited in wild. Thus, Canonical security team has decided to disable the plugin in an update taking into consideration the severity of these issues.
In near future all Sun Java packages will be removed from partner repositories. You can read more about the announcement from here.
While this is certainly a right step, many Minecraft players won't be happy as they prefer Sun Java packages over OpenJDK.