Well DOA4 does not carry an "Xbox Only" exclusivity badge...
Then again neither did DOA3
Despite the fact that the PS3 will have a larger fanbase behind it... it's also a more saturated market for fighters... DOA would have to compete against Tekken and Virtua Fighter, where it really doesn't have much or anything for compeititon on the Xbox... Not to mention DOA in general (more-so with DOA4) is geared more towards fans of the series then casual gamers. With that in mind... fans of the series are more likely to own MS hardware...
I think if it does hit PS3 at all, it will only be in Japan.
Both DOA1 and DOA2 were released cross platform in japan on the Saturn and PS1, then on the DC and PS2 respectivly...
DOA2 hardcore came after the initial release of DOA2 on the PS2 in japan... DC also go an updated version of DOA2, "DOA2 Limited Edition" that was essentially the same thing as hardcore, it never hit US shores because of the dying DC market here.
DOA3 was the first game in the Series to NOT get released multiplatform. and MS bought exclusivity of DOAX, and DOA2U, and Ninja Gaiden Black (hence the "Only for Xbox" logo on the cases")