I don't think that the investment on the driver side can be that much for a giant semiconductor company. Because I think patching an already "working" fglrx driver for a new kernel and X mustn't be a trivial job (and nvidia does it quite good). And because the risks and amount of care that must be taken is not on the driver side but on the hardware side. Afterall they can develop the drivers and make them better but if they make a faulty chip and produce it then.. holy cow!OEMs (nVidia, AMD, etc.) claiming they need more users/active game devs/publishers before they invest more money.
So as an ex-R500 user, I think its rather a policy of amd/ati to give sorta linux support to linux user customers.