We've heard you! Yammer is changing the posting shortcut from Shift-Enter to Ctrl-Enter / Cmd-Enter (standard behavior across most other productivity and communications tools).
When we first introduced this shortcut, Yammer was following what seemed like an emerging standard. But the standards changed! So now Yammer is changing as well. In order to better align with user expectations, Yammer is adopting this shortcut across the product. Shift-Enter will insert a line break (in both regular posts and chat windows).
We are planning to roll out this change very soon. Let me know if you have questions or comments!