It's a two-step process. First, since the Tobacco Act did not include e-cigarettes (and several other tobacco products), the FDA would need to issue a "deeming" regulations stating that the Secretary deems e-cigarettes to be included in the products it will regulate under the Tobacco Act. There would then be a public comment period, after which the FDA would issue a final rule on which products it deems to be under its purview.
Second, the FDA could begin issuing draft regulations that affect e-cigarettes. Each proposed regulation must be posted on Regulations.gov and allow a public comment period. After that, the FDA would issue the final rule.
So what we are talking about is not a "ban" per se. However, the FDA could issue regulations that result in a de facto ban. For example, if they go forward with applying section 905(j) and announce that all e-cigarettes that were not on the market in February 2007 must be taken off the market, that would eliminate 510s, KR808s, eGos and similar devices, Mods, etc. That could not happen until the FDA first issued a deeming regulation, and second issued a specific regulation affecting e-cigs.
I have heard rumors that the FDA will issue the deeming regulation next month, but it could happen next week or it could happen many months from now.
The public comment period for each proposed rule or regulation is usually 60 days. It can take several months after the comments close for a government agency to issue a final rule. So the short answer is "not immediately" and the long answer is anybody's guess.
