If I send a test message to gmane.test[1] using Thunderbird, and then
view the message with Gnus, the characters are displayed correctly when
visiting the group. On the other hand Gnus displays the message
incorrectly in my "Sent" folder:
,----
| User-Agent: Thunderbird 1.5.0.14ubu (X11/20080306)
| MIME-Version: 1.0
| Newsgroups: gmane.test
| Subject: test
| Content-Type: text/plain; charset=UTF-8; format=flowed
| Content-Transfer-Encoding: 8bit
| [...]
|         character: ä (4194276, #o17777744, #x3fffe4)
| preferred charset: eight-bit (Raw bytes 0-255)
|        code point: 0xE4
|            syntax: w 	which means: word
|       buffer code: #xE4
|         file code: not encodable by coding system windows-1252-unix
|           display: no font available
`----

Also other "normal" Emails are displayed incorrectly. Example:
,----
| User-Agent: Thunderbird 2.0.0.12 (Windows/20080213)
| MIME-Version: 1.0
| Content-Type: text/plain; charset=UTF-8; format=flowed
| Content-Transfer-Encoding: 8bit
| [...]
|         character: ä (4194276, #o17777744, #x3fffe4)
| preferred charset: eight-bit (Raw bytes 0-255)
|        code point: 0xE4
|            syntax: w 	which means: word
|       buffer code: #xE4
|         file code: not encodable by coding system windows-1252-unix
|           display: no font available
`----