I try to minimize how much I hold my phone in my hands in public for this reason sure (It also makes you a more viable target for muggers if you look distracted by your phone). You can also set TextSecure to automatically wipe your passphrase from memory when you're done reading/writing texts. I should probably also be using APG ( https://f-droid.org/repository/browse/?fdfilter=APG&fdid=org... ) to encrypt other stuff for the same reason.

I don't doubt that this is a real threat for sure, but there are measures you can take to prevent it from happening.