Well... well... I hope you use Eclipse Helios, because what you asked is available on Helios.
Put your text cursor again on the method and click menu Navigate ? Open Implementation. Now if you have more than one implementation of the method, you will get choice to pick which implementation to open.
By defining a keybinding on Preferences ? General ? Keys you can even use the feature easier, but before you do that, see if this shortcut is fast enough for you.
Press Ctrl + click and hold. Now move your mouse over the same method. Tadam… you will get choice.
If you pick Open Implementation you’ll get the same choice as before.