Hotmail Wave 4 has begun its worldwide rollout, right on schedule.  However this rollout is being accomplished by server cluster, and not by region or individual accounts, as Mike Shackwitz explains on this morning’s Inside Windows Live blog post:

our Hotmail servers are grouped into clusters, and we have hundreds of clusters. We start by upgrading a single cluster, wait to make sure that it’s running smoothly and that we didn’t miss anything during testing, and then we continue on to the next cluster. If we find a bug, we’ll fix it before continuing further, and give that fix to everybody who has been upgraded so far. That way, we release the highest quality code as we proceed, and we can minimize downtime for everybody. Accounts are assigned to clusters to best balance our total account load, not by country or by tenure of the account, so each account is as likely as the next to be upgraded. Given this, we can’t move any individual accounts up or down the migration line.

A number of you have been asking in the comments about Exchange ActiveSync, and we wanted to clarify by way of a couple of quotes, first from the Hotmail Fact Sheet on windowslivepreview.com:

Hotmail supports push e‐mail on the Web, PC and now also on the mobile phone. Using Exchange ActiveSync, available on nearly 300 million phones, with the new Hotmail you can not only seamlessly synchronize e–‐mail between your phone and the Web, but also synchronize your calendar and contacts.

And then from Paul Thurrott, who explains it all better than we could:

First, Hotmail is not switching to Exchange. It is still its own unique thing.

What’s happening is that Hotmail is adopting the Exchange ActiveSync protocol so that it can sync email, contacts, and calendar data, over the air, to mobile devices like smart phones.

If you want to access the new Hotmail from Outlook, you will need to continue to use the Connector software. You cannot configure a desktop email client, like Outlook, to access Hotmail using Exchange-type server data.

If you want to access the new Hotmail from a third party email program, or a Mac-based email program, only POP access is available, as before.

(Thanks to jvd897 for the link, and you can read more from Paul in his Hotmail review).

UPDATE: Thanks to a tweet by Microsoft/Hotmail employee Ryan Burk, we’re reminded that the ActiveSync integration with Hotmail will be a separate rollout, and “it’s not part of what you get today”.  Thanks Ryan, we will be watching!

Now on to the rollout itself.  We’d like to try and track how many of our readers are getting the new Hotmail, and so for the next few hours/days, how about this: if/when you get the new version, send us a tweet to @liveside (and remember to follow us for the latest in Windows Live news and more!) with the hashtag #NewHotmailHere!  We’ll monitor the @replies, and hopefully see a flood of reports coming in soon!  Of course we always welcome your comments here, as well.