That's very good!
cPanel doesn't seem very confident about IPv6 implementation as they couldn't even give a timeframe for it. cPanel will lose its market share if it doesn't pull its socks up. Just because it's the current leader for control panels doesn't mean it can take its own sweet time for IPv6 and force their customers to wait for it.
Actually, this is a very good time for another competitive product to enter into the web hosting control panel market and become the new leader...and I hope it's open source!
How about setting up a separate server to explore cPanel alternatives with IPv6 compatibility? Just in case cPanel doesn't deliver, you can quickly switch to one that does and still keep your customers. You can invite some of your existing customers to try it out as a means of testing it in a real world scenario. Free, of course!
Here are some cPanel alternatives supporting IPv6 as shown in Wikipedia's comparison of web hosting control panels :
SysCP
Virtualmin / Virtualmin Pro
Webmin