I appreciate that this may be impossible to debug remotely, but I have one strange anomaly to add to the issue: I was later able to connect to the same Mac when the iPad was on the same local network (same subnet) as the SSH host. It could have something to do with the routers involved in my outside-the-firewall connection.

I ran into this issue today. I've tried Textastic with two hosts: one Linux (which worked fine) and one Mac OS X v10.7 (which caused this error). The log messages are identical to the above.

Looking at the Mac's log, I saw this in the /var/log/secure.log file for each attempt I made:
Did not receive identification string from x.x.x.x
(where x.x.x.x was my iPad's IP address)