Due to an acute shortage of desk space, Kim and I decided that we were going to run our M1 Mini headless for the time being. It mostly just handles iDevice backups and iTunes libraries, so no great loss and we can talk to it via screen sharing.
I moved the Mac to headless and it responded to pings, but didn't have any services running. Odd.
Eventually after consulting with Gaige I dragged a keyboard and monitor over and plugged them in. Login window. Hmmm. Clicked my account, typed my password... and then everything started working across the network.
You've probably read in the trade media that the M1 is fast. Astonishingly fast. Not only that but with nVME disk, it boots fast too. Fast enough that it doesn't necessarily stick in your mind that you're typing your password at the FileVault login screen rather than the MacOS login screen.
Upon further inspection I discovered that FileVault was on for the boot drive. Turned it off, rebooted, and all was well.
It came to me as a complete surprise that the Mac would DHCP and respond to pings before booting from a FileVault-encrypted drive.