Thanks for the information about the email and the case-sensitive username. I've changed the signup application to send the username in all lowercase, and to note that it is case sensitive.
It's about the initial subdomain becoming active. Basically, Apache needs to restart before new vhosts are synced, and your site won't appear until it has a vhost. An Apache restart results in about three minutes of downtime, so we do it just once a day.
And yes, you can indeed see your site modifications in real time. Every time that you request a page from Stevie (the HelioHost server), Apache calls mod_include (SSI backend) to parse the includes. That means that changes are reflected immediately.