I went to a local entertainment store recently and PS2 is doing way better than X-Box here in Au8stralia.
Also I spoke to a Bungie employee online about a week ago and he said if Halo is really sucessful there will be a sequal.
And the "only on X-Box" thing is wrong. There has been comfirmation for PC versions of many X-Box games. Halo being one of them.