Q: Are all iMessages encrypted end-to-end, on every phone provider plan?

A: If sending from iMessage to iMessage, it is encrypted point to point. However, Apple holds a master key, and is able to read all iMessages.

It was recently reported that Apple does archive iMessages for between 1–3 months.

For true point to point encrypted messaging, without a middleman able to recover your messages, I recommend wire.com.