In many situations you would like to program Arduino without any cables. For instance while testing mobile robots, or simply when your board is installed in not convenient position. So why not setting up an inexpensive WiFi connectivity to program Arduino on the run. Oscar have used ESP8266 based WiFi module.
He didn’t try to rewrite Arduino bootloader to work especially with WiFi module, but instead he wrote a processor program that listens for specific commands on coming from WiFi module. Once “reboot” command is received, Arduino resets and enters its stk500 bootloader. On PC side there is a Python server running which streams new firmware to chip. After software is uploaded, Program continues normal execution. [translated source]