Do the following (you will need a memory card).
Go to the [PLAYER MENU] screen and choose [NEW GAME].
Enter [GYU GYU ???] where ??? can be any three letters you want.
Press the [X] button on the controller. If the cheat has worked you will be moved back to the VM menu.
If you go to the [LEVEL SELECT] screen you will find that ALL of the levels have been unlocked. You may now select any level you want.
(You cannot create a save file with all levels unlocked this way though, you need to finish the game by actually making your way through to do this)
Do the following (you will need a memory card).
Go to the [PLAYER MENU] screen and choose [NEW GAME].
Enter [GYU GYU ???] where ??? can be any three letters you want.
Press the [CIRCLE] button on the controller. If the cheat has worked then a plant on the left side of the screen will release a huge stream of bubbles.
Press the [X] button to create the save file, then return to the main menu.
If you go to the [LEVEL SELECT] screen you will find that ALL of the levels have been unlocked. You may now select any level you want.
(Thanks to Smidsy for the great descriptions!)