Ok so this post is for anyone who has updated to 1.3 but wants to still be able to play opticraft ;). If you click the link below it will take you to a mediafire page from wich you will download a file called "minecraft.jar". Once you have downloaded this read the instructions below the link.
http://www.mediafire.com/?f0zpov99n1nk1xk (http://www.mediafire.com/?f0zpov99n1nk1xk)
INSTRUCTIONS
1. Click the start button and search %appdata% and click on some thing that will Say roaming.
(http://www.opticraft.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FZb7h3.png&hash=ab46c2c94872066b6dc05e8d36d8e6d8)
2. Double click on a folder called .minecraft and then double cklick to a file called Bin.
3. Delete the file called minecraft from the bin folder.
(http://www.opticraft.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2Fblq3F.png&hash=e49f7ed05c0db96bf280b4df060216bf)
4. Now copy the file you downloaded from the link and paste in the Bin Folder.
(http://www.opticraft.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FyG8Vv.png&hash=3ef2ec1cbb9693a45937686743fc8af1)
5. Now your client should be a minecraft 1.2.5 Client.
(http://www.opticraft.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FRbi5Z.png&hash=3f62b4677e81475820a8d3baa2730767)
6. When starting the game decline the update.
(http://www.opticraft.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FKKwyQ.png&hash=81e7e2b59027ebf1a2b3fd2dea2b2d87)
You should now be running 1.2.5 on your client. :)
(http://www.opticraft.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FMQPQg.jpg&hash=a901d5d455171084c6d03b4bf2f7a8ca)
EDIT: Added Some Pictures ;)