Basically, this setting has to be set to "utf-8", or else output of xml based datatypes (ezxmltext) may fail:
FILE: setting/override/i18n.ini.append.php

[CharacterSettings]
Charset=utf-8

This may cause some confusion among those who work on the upgrade task, and can be a bit tricky to sort out, if you skim to fast through the upgrade doc and miss the one sentence where i18n.ini is mentioned.

the current upgrade docs:
http://ez.no/doc/ez_publish/upgrading/upgrading_to_4_0/from_3_10_x_to_4_0_y

Section:
>Step 2: Upgrading the database
>>Switching to UTF-8

The current text mention this setting, but should also include it in the final comment.
Current:

After running the script, make sure that eZ Publish knows about the database's new character set by editing the setting mentioned above:
Charset=utf-8

Suggestion:

After running the script, make sure that eZ Publish knows about the database's new character, and also update the eZ Publish internal character set accordingly, in the correct override files:
FILE: site.ini
[DatabaseSettings]
Charset=utf-8

FILE: i18n.ini
[CharacterSettings]
Charset=utf-8

I set 4.3.0 as affected, since it may be the destination of the upgrade path.