A little more info; I have both an RS Pro and a RB450G.
The RS Pro initializes the AR8316 in Redboot first thing:
AR8316 PHY found on MAC0
AR8316 PHY found on MAC1
Ethernet eth0: MAC address 00:15:6d:c1:3b:fb
IP: 192.168.1.20/255.255.255.0, Gateway: 0.0.0.0
Default server: 0.0.0.0
RedBoot(tm) bootstrap and debug environment [ROMRAM]
Ubiquiti Networks certified release, version 0.9.00318M.0905121200 - built 12:09
Copyright (C) 2000, 2001, 2002, 2003, 2004 Red Hat, Inc.
Board: Ubiquiti RouterStation PRO
RAM: 0x80000000-0x88000000, [0x8003bf00-0x87fe1000] available
Flash: 0xbf000000-0xc0000000, in 256 blocks of 0x00010000 bytes each
== Executing boot script in 1.000 seconds - enter ^C to abort
OpenWrt then mostly works.
The same OpenWrt image does not work on the RB450G, even though the hardware is virtually identical because (apparently), Mikrotik's RouterBoot initializes the hardware differently and then RouterOS must do some stuff on its own.
When you use network boot to boot the RB450G, RouterBoot seems to configure the switch for DHCP and off you go. What we need for the RB450G is to figure out what the initialization code is that sets the switch in a state so that OpenWrt can run with it.