Bluetooth Serial example un Puck

Some more questions: is there a way to actually detect that there is a BLE uart connection present at all?

As @AkosLukacs says you can detect connections, and NRF.getSecurity().connected allows you to query if you're connected - however you can't see if someone is connected but not looking at the UART.

Just a note that the Web Bluetooth Puck.js lib (http://www.puck-js.com/puck.js) now supports flow control.

HOWEVER while sending binary data over UART was fine, with XON/XOFF flow control (now the default in 2v05+ builds) you can't do that. You'd have to explicitly disable it using Bluetooth.setup(...) if you want binary.