Not enough information is available about this yet for me to want to cover it in TidBITS proper, but I think the warning is important enough to share here. In short, using PGP tools in email could pose a risk, including potential exposure of previous messages.
@joe has researched this and written a blog post about it. In short, the sky is not falling. 
https://www.takecontrolbooks.com/blog/what-you-need-to-know-about-the-efail-vulnerability