Piracy should help out the PS3 a bit (but will probably start eating into that system's software sales) so there might be a few weeks here and there where the PS3 is ahead but from here until at least the end of 2011 expect the 360 to be ahead the majority of the time.
If you're talking about specific regions I think the 360 will always be ahead in North America in monthly sales until the end of this generation.
Europe is hard to call but I think that even if the PS3 starts outselling the 360 again in that region it'll never be by the numbers it once was.
Japan is the weak link.. If the 360 was as strong as the PS3 in that region I would have said that the 360 would outsell the PS3 for the rest of this console generation but it's the only thing that will keep the PS3 from falling further behind and might allow it to catch up slightly in the next 2 years.
