Sometimes, when I open an InfoPath form template within Visual Studio, and go to the Data Source panel, the panel comes up blank. I have seen it do this many times over the last few months, and hadn't figured out what causes it. I did learn that the panel wasn't entirely blank, because if you drag the panel to the left and expand it to take up the majority of your screen, you can see the right-half of the Data Source panel (a lot of good that does!).
Anyways, I still haven't nailed down what causes this situation (it seems to be more of an issue if I have 10+ windows open, makes me think its a memory problem). But I have determined that I can fix it by closing out of Visual Studio, going to the solution folder and deleting the .suo file. Then, when I go to reopen the solution, the problem is gone. Just something to remember if this problem becomes an issue for anyone else!