From what I hear PS3 users won't be able to play with PS4 users. This doesn't just count for COD Ghosts. If I remember correctly it's because they will be on different PSN servers. Anyways, so long story short PS3 users won't be able to play with PS4 users on Call of Duty Ghosts. Call of Duty/Activision does have some deals for upgrading to the next gen when you're ready here: http://bit.ly/1dBIE2k
Hope this helped!
Keep on gaming!!!
No, cross platform is not going to be available as it is not now regardless of PS3/4XBOX/PC. Iam just curious to see if we get screwed further as we already do getting the DLC a month later than XBOX, Microsoft and PC will have dedicated servers for Ghosts and nobody will confirm if it will be the same with Playstation. The servers will actually be hosted by Microsoft/individuals (in the case of PC). I feel with having to pay to play online with the PS4 that we should have dedicated servers as well.