mb_send_mail() does not send UTF-8 encoded messages properly

I am trying to send UTF-8 encoded e-mail message with mail() function,
overloaded by mb_send_mail(). The following message is recorded in error
log:

PHP Warning:  mb_send_mail() [<a
href='function.mb-send-mail'>function.mb-send-mail</a>]: Unsupported
charset &quot;&quot;utf-8&quot;&quot; - will be regarded as ascii in {file}

The message is sent ASCII encoded and is garbled.

mb_language returns 'neutral' as it should be for UTF-8.

I'm using PHP 5.1.6 / Apache 2.2.3 on Debian testing.

Anastas