All jokes aside,
The generator would likely see practically a dead short. Depending on the circuit protection on the generator, or even the fuse that connects to the outlet, that may pop first. But a portable generator would really complain about being expected to liven up half a neighbourhood.
I'm not convinced that the voltage applied would be very high, without doing any math: you know, the number of houses connected verses the impedance of the alternator. Many more amps than what the fuses are rated along the way.
But yes, before anyone complains... Pretty dumb to do that when the are safer options. Please don't plug generators straight into your house power outlet. Ask somebody to install the right switch gear so you can do it safely.
To be honest, I would be much more concerned with the linemen that may be zapped by a generator trying to feed the entire neighbourhood.
Linesmen always treat all lines as live, always. The reason is the wires can become live unexpectedly for many reasons such as falling trees or even in case of miscommunication among repair crews.
A big danger would be connecting a running generator to live grid, not realizing it's live. Wouldn't like to be standing anywhere near the generator.