You know how in Eclipse, pressing F3 over a method will take you to its declaration? Well I have a method that is part of an interface; clicking F3 ove
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.