I have exactly the same problem. I can use eclipse to compile->build->upload the "blink" example.
However as soon as I try use "Serial", the compiler gives the following error:
Symbol 'Serial' could not be resolved main.cpp line 10 Semantic Error
I use the library that arduino ide generates (copied from /tmp). I also included every .h file that I found under / directory to the eclipse project. But still I get the same error.
Does anyone know the reason for this? How may I solve this problem?