At least no in the strictest sense. SCEA executive VP Jack Tretton has stated that,"It's very difficult to draw analogies [between] your success on SOCOM and your success on Madden. Madden players care about peoples' success on Madden."
What does this mean? Cross-platform games that also appear on the Xbox 360 may carry over their achievements to the PS3 in some form and that each game will have its own personal ranking system, instead of a cohesive "Gamerscore" which combines points garnered from every game a person has played.
This could be a good thing considering that, as Tretton points out, it's hard to hold achievements in games of two vastly different genres to the same standard. 360 gamers have already complained that some games' achievements are far too easy to complete, while others are nearly impossible. Microsoft currently has no way of reconciling this and it dilutes the overall purpose of having a Gamerscore in the first place.