For years my favorite Android app for use with RTM has been Bionic Cow (sorry Bob). Recently I contacted the app author to see if he planned to add sub-task functionality. Sadly, his response was, "Unfortunately I'm not able to actively develop and improve the app. I recently released it as open source project so that anyone interested can develop it in the future (https://github.com/gpini/bioniccow)."

I thought I would post this here in case anyone might be interested in picking it up and further developing it.

FYI - it is a 4.3 star app and I'll bet I'm not the only one that would pay a reasonable fee for a working update that includes sub-tasks and the current "folder" option.