Currently pressing enter in the telepathy chat ui sends the message instead of entering a line break and AFAICS there's no way to change that. Please add send message to the list of configurable keyboard shortcuts so users can change it to the standard Ctrl+Enter or something else.
ktp-text-ui accepts shift+enter to enter a line break. That shortcut is quite standard and it is used by most of the other clients
Also the numpad enter key enters new line, not sure if that's a feature or bug though ;) Btw. re-closing as "wontfix" as we don't provide custom shortcuts for the send button and apparently won't be (though patches are welcomed), so closing as "worksforme" is technically not correct ;)
FYI, this has been implemented in 0.6.