I'm a web developer as well and it can really go either way in terms of the site being down. I typically do what elfshadow mentioned (just working on pages that are at a url that nobody knows, but could still find if they tried hard enough), then again it's possible to work on areas of a site while restricting access to them on the server side.
Typically I try to keep the main or old site up because there are a lot of users viewing it while I work. Sometimes however it's unavoidable to have some down time like in the instance of changing domain names or some major functionality. With a domain name switch for instance the site will be down for a short time up to several hours while the new address propagates around the web.
I think ultimately with how long it has been down it must either be lingering server issues from the unauthorized access that BlueMue mentioned, or a deliberate taking-down to work on it... so we've got another nice 50% chance of good evidence! ;) I'll try to snoop around a bit but I doubt I'll find anything. Plus with the posts I've read on here it looks like we already have a full force of Shenmue Super Sleuths on the case ^^. It's actually kinda touching seeing everyone pooling their skills to figure it all out lol.
**Sega updated their status page today or yesterday with the latest list of sites to come back online. Here's the url if you'd like to look at it:
http://sega.jp/topics/140523_0It's very likely shenmue.com is just down from this issue. Here's the horrible translation of the latest text on that page:
"By the unauthorized access to our web server, which is confirmed in May 12, we have to stop the viewing of some official site today.
With regard to the official site that you stop reading, we have resumed in sequence from the site of restoration work has been completed from May 16.
For the official site was resumed, and we have been stopped some content and services for current situation, restoration of tentative.
※ sites that we have resumed at the 19:00 time May 23 will be the following.
also with regard to the site still has not been restored, the work upon completion, will resume in order.
To everyone, it has the inconvenience continue, I am really sorry.
We ask that until full recovery, you please wait for a while."