On the main menu on the second 'Bonus Material' disc, highlight the back light or the cone. When the Polymorph turns into the rabbit, press up to select the skutter's board. This only appears for a short time, so you will have to be quick. If you do it successfully, you will see an animated interview with Rob, Doug and Ed talking about Polymorph.
Go to the 'Episode Select' menu on the first disc, and let it play through two times. The music will change to the party music from The Last Day and the Polymorph will arrive on the screen. When the Polymorph changes to the rabbit, select him by highlighting 'Drive Room' and pressing down. Select him, and a question mark will appear over his head, now you should press select again. Like the other Series 3 Easter Egg, you must be quick. You will be shown an interesting short scene about the effects of the Polymorph.
In the Photo Gallery on the second disc, go to the ninth picture in the 'Models and Covers' section (the first Starbug design sketches) Press Up and the picture will become highlighted. Select it and you will be shown a behind-the-scenes look at the making of the Starbug toy.
All easter eggs were generously submitted by alsotop