If you don't already have access to the MoP Beta, we have keys for you. Go sign up for one. (Not a contest. If you qualify, you get a key. Keys #001-#186 assigned so far.)
There is quite a bit of information about the UI API and graphics changes already posted in our MoP Beta API and Graphics Changes forums that you are welcome to check out. Further contributions are always appreciated.
We also have the in-game guilds set up on both the EU and the US beta servers again. If there is no one in-game to give you an invite, hop into the IRC channel irc://irc.freenode.net/WoWUIDev to see if anyone is on that could log in quick for you.
One of the changes we’ve made to the site this year will give you another way to deal with your beta versions. Now you have the ability to ‘tag’ your addon for more than one download category. So if you think that your live version will also work in the beta, you can just add the MoP Beta category as an optional category. (Make sure that you are tagging it for the MoP Beta cat, not the regular Beta cat.) If, however, you think that the one for beta is going to be incompatible on the current live game, you can upload a completely separate file to the MoP beta category as per usual.
Once you've uploaded them for testing, be sure to check back for any bug reports and other feedback users may provide, whether in the comment field on the download page of your addon, a feedback thread you have created in the Released MoP Beta AddOns forum, or in the MoP Beta AddOn Help & Feedback forum.
If you don't already have access to the MoP Beta, enter our contest for a chance to win a key.
As mentioned above, we've got our MoP Beta forums and a download category set up for you to use. There's already 25 addons that have been released/tagged for testing in MoP, with more being uploaded all the time. As you check them out, please remember that these are beta versions of these addons, being tested in another beta environment. There is a very good possibility that many of them may have bugs. Give the authors feedback letting them know any bugs you find. You can post it in the comments on the download page for that addon, in the discussion threads for the addon if the author has created one, or make your own thread in the MoP Beta AddOn Help & Feedback forum.