Along with Mountain Lion comes Messages, a replacement for iChat. You can download Messages now. When I installed the Messages app, I noticed right away that it would no longer accept text invitations automatically. In other words, when I get a chat I first have to accept it and then I can respond.
Luckily, when iChat was replaced, it didn't wipe out the Scripts folder. So, you can still use the iChat script in Messages.
Here is the setup:
First you need to go to the "Alerts" pane in the Messages Preferences. There you will change the drop down list to "Text Invivitation" and choose to "Run an Applescript script". (I like that sentence. It's like it's from the office of redundancy office.)
Anyway, when you choose that you'll need to find the script at /Library/Scripts/iChat/ and apply the Auto Accept one. (You may need to hit Command+Shift+G to open that location)
When you do this, it will also create a new folder and move that script to a new location at ~/Library/Scripts/Messages/.
Glad to have that back.