This was a bug with the preview, and it should be fixed now.

It should be safe to type those characters in comments and everywhere else - your post should not get cut off. I just tested by commenting a friend with multiple hearts and it worked :)

Friend feed page has now been fixed as well :)

Anywhere else you've noticed, before I lock this up?

Hm... I know it's related, but I'm actually going to consider sMail a separate issue as it has to do with larger problems related to sMail. Eventually we should give it a revamp - due to how complicated the code is, there is no quick fix for that without the risk of breaking older sMails unfortunately.