As I understand it, an OPTIONAL guest registration upon checkout is not currently available. VERY glad to hear you're working on it, as this is rapidly becoming a problem with one of our hosting customers using Hikashop.
I have successfully configured Joomla user registration to activate accounts instantly, and am relying upon HikaShop's two-step purchase process to discourage automated orders being placed (This has been successful while we ran the store without requiring registration upon purchase, so I'm not terribly worried about that.)
By setting Joomla to instant account activation, we're avoiding the incredibly awkward interruption during ordering that Joomla's confirmation-link email interaction used to cause, but creating the user account during checkout is now a one-step process that occurs before purchase completion, so we'll could see an uptick of junk accounts created by spambots - even though the bots probably won't attempt to complete the second step in order to successfully place orders.
One thing I have verified is that HikaShop's member registration plugin and template seem to override Joomla's default registration form, which isn't much of a problem, except that the HikaShop template doesn't seem to support Joomla's built-in support for ReCaptcha.
Is there any way to make this happen? Though we haven't activated this configuration yet, we'd definitely like to take steps to avoid the creation of junk user accounts, which even a simple captcha is generally good at.
Also, is there any timeline yet on the optional registration-upon-purchase?