It's easy to add Dynamic Content to the Subject Line of your email...
IF you know these two things.
1. Keep the Dynamic Content text only.
2. In order to drag and drop your content into the Subject line, watch your cursor. What do I mean? If you try to drag the actual content on top of the Subject Line field, it doesn't stick. (See screenshot and notice that my mouse is ABOVE the Subject Line Field.)
Instead, watch your cursor as you drag and drop the content. As soon as your cursor is inside of the Subject Line it changes from an arrow to a line. Then release your mouse and voila the Dynamic Content sticks in place. (Notice in the screen shot I have circled my cursor which has changed from an arrow and is inside the Subject Line field. Even though the Content appears below, when you release the drag and drop the content will now stick in the Subject Line.)
Go forth and be dynamic!
Also, just as an FYI dynamic content that references Custom Objects will be a part of an upcoming Eloqua University class. If you are attending Modern Marketing Experience London in a few weeks look for the Effective Marketing with Custom Objects class on the Continuing Education day. The London class will be a two hour version. A three hour version of this course will be available for All Access Pass holders soon.