Since installing Juneau, I've had various issues accomplishing even the simplest of tasks. I would get errors saying that the SqlCeManager could not be initialized or "object reference not set to an instance of an object." Depending on what I tried to do I might also see a big error dialog that suggested installing the fix from KB #974247, however this fix did not work. What ended up solving my issues, and allowing me to actually demo the new code-aware table designer in my Denali session tomorrow, was KB #2553608. You'll need to go to this link:
http://support.microsoft.com/hotfix/KBHotfix.aspx?kbnum=2553608&kbln=en-us
Check the box, fill in your e-mail address (twice), decipher the captcha, and request the download. Once it comes in, extract the archive, and run SSCERuntime-ENU.exe. This will extract SSCE_Runtime-x86.msi and SSCE_Runtime-x64.msi. If you are on a 64-bit system, you'll want to install *both* MSIs. I installed the x86 version first, then the x64. Once I did that, I was able to successfully connect to databases, create projects, etc.
Hope this helps.