On PC's MS Outlook Express also works very well with Unicode - though some people have security concerns about it.
day before yesterday i had to test a few email clients and webmail for unicode support and:
the email list "view" in OE doesn't support unicode for the subject (known issue, http://support.microsoft.com/default.aspx?scid=kb;en-us;324603). the preview pane & the email itself (ie double clicking to read the email) does though. and as you've said, thunderbird works plenty fine. using firefox 1.0, gmail works fine. hotmail works though hotmail has no obvious encoding hints and you have to manually set the browser's encoding. interestingly, the "today" page comes up as utf-8 encoding but the email list and email pages come up iso-8859-1.
