Do I need SSL for "Coming Soon" email signup form?

The rule of them is if you are sending or receiving sensitive information you should be using encryption. What is considered sensitive is subjective but anything considered private or personal can be considered sensitive (e.g. passwords, credit card information, social security numbers in the US). Email addresses are not generally considered private or personal so in your case an SSL certificate wouldn't be necessary.

I agree with @John, as long as your site is COPA compliant-- if applicable.

Email addresses, according to COPA, would be considered sensitive information as it could be used to contact a child online. Similar rules may apply to HIPAA or other international regulations (I really do not know). Food for thought though.

http://business.ftc.gov/documents/bus45-how-comply-childrens-online-privacy-protection-rule

Hope this helps!