After much digging about on the forums, I found that it's been hidden. You can re-enable by going to Settings -> Misc -> Enable Third Party Add-Ons. Then restart Astrid (may have to reboot Android).
The reason this change has been implemented is that support for Producteev is being phased out. This caused me some concern; I email my tasks to keep a separation between "todo" items and communication emails. However, unfortunately for Producteev, Astrid now do this in-house. Email to: email@example.com and it goes on your list. They have similar natural language features like: "Get haircut (tomorrow)". I'm going to give this a go but looks likely that I'll switch.