I ran into the same issue earlier this week, wanting to use the rMP3 shield with Arduino 1.0 without have to deal with the core update. What I changed is the second include and the object initialization, since NewSoftSerial is now SoftwareSerial in 1.0.
SoftwareSerial rmp3_serial(6, 7);
The RogueMP3 library V0003 requires a mini update as the Serial.write() now returns "size_t" instead of "void" with Arduino 1.0. To fix, you can edit line 155 in RogueMP3.h and line 518 in RogueMP3.cpp, changing "void" for "size_t".
It's all working fine here otherwise. Let me know if you can't make it work on your side.