AFAIK most mice/keyboards still ship with PS/2 adaptors. Even my brand new Logitech Elite Duo has them.
So I don't think that will be a problem at all. The only issue with the PS/2 mouse adaptors are that all those extra buttons beyond left/right/middle/scroll are rendered useless. Optical mice shouldn't be a problem at all. There are still people that swear the polling rate of PS/2 mice is better and thus more accurate than USB mice.
actually, the extra buttons still work regardless of your connection. The maximum image read on the position of your mouse is 200 times per second (at least, that's what the mouse accelerators went up to; 80 was default) . A USB optical mouse reads at 1500+ times per second, and can be sent to the hardware. I doubt the PS2 port used is going to translate that to the Xbox. Who knows.
As far as the adapters are concerned, I'm well aware of them. I've got 4 intellimouse opticals (different versions of the explorer, both wired and wireless) so I have at least 4 of those ugly green adapters. I used to have to use one of them at work as the desktop standard was NT4WS which doesn't properly support USB.
See, the thing that made me buy it (got mine on preorder with Lik-Sang) is that I want to see how the configuration works. The Xbox buttons are limited, and I'm pretty sure the Xbox isn't going to play nice with a full keyboard. I'm thinking an Fkey is going to bring up an overlayed config window for us to choose which buttons do what. That also means it may allow addn'l buttons on the mouse, as long as the driver used on the adapter supports them.