Use DHCP; find server via broadcast; cleanup
Fix initial LED blinking.

The pattern is now quite useful, and follows these steps:
0) All LEDs are off to begin with (as there is no power! :-)
1) Power on: ON is lit up (it is hard-wired between +5 V and ground, via a resistor, so it will always be lit when there is power)
2) Net starts blinking, while setup() tries to acquire an IP address via DHCP (it loops until it succeeds, even if forever)
3) Net stops blinking when DHCP is finished, and is steadily lit
4) Status starts blinking as setup() sends a PING out to the server (via broadcast)
4) Status stops blinking as the server has replied (retried forever), and remains *unlit*
5) Initialization complete! After this, net is always lit, and status blinks once every time a temperature reading is transmitted.

I realized a potential problem in the above: while waiting for a server reply, it's not impossible that a DHCP lease times out, etc...