First I must declare a clash of interests - I am a long time Thunderbird user.
I can see problems with there not being a Mail client included OOB with windows 7 but also I can understand why this action has been taken.
A lot of, especially novice, users consider things such a IE and WM as the operating system and I have had more than one occasion where a problem with IE or Outlook Express or Windows Mail was explained as "I've broke Windows". These users are going to have a major problem if the lack of an integrated client is badly handled.
The reason for this ommission, is of course an attempt to head off the monopoly regulators in various markets, who have already stated that they will agressively investigating the release of Windows 7.
I personally would like to see the presence of a mail client is Windows 7 RTM or at least an explanation at the first boot after install providing links to the Live suite, whether this needs to go to the lengths of " other Mail clients are available" would depend on MS and the regulators.