Twice today I have tried to reply to a post using the Reply With Quote button, only to have it dissappear. FYI, I am using IE7.
I'm having a nightmare dealing with people because they're replying with no quoted text. Everytime someone PM's me I have to back track through the old messages. It's making buying anything take 3 or 4 times as long. Extremely frustrating. Not sure if people are just deleting the message thread on purpose or it's the IE issue you have outlined.
Click the "View History" link under the PM and you will see all the PMs back and forth you had for that topic.