If you want to use Eclipse without copying the core.a file, you can link the arduino library files into your project.
To do this, go to File>New>Folder. Select your arduino project as the parent folder. Name your new folder (I called mine ArduinoLibrary), then click Advanced>>.
Check "Link to folder in the file system" and browse to:
(Note that /path/to/arduino is the path to the location where your copy of the latest arduino ide is installed. for me it is /Applications/ardunio0014).
And click OK.
Now your eclipse project will compile all of the necessary arduino files when you build. However it will also try to compile main.cxx, which is the file in which the Arduino system defines its main() function. I can't figure out how to set up eclipse to compile this file AFTER it sees my arduino sketches, so my workaround is to do the following:
Right-click on main.cxx and select Exclude From Build...
Select all and click OK
Now you must manually do two things to get your arduino sketches (sketch = *.pde file that contains definitions of setup() and loop() functions) to compile:
at the top of your sketch. Then copy the entire contents of main.cxx into the end of the sketch.
Oh! And you have to rename your .pde files to .cpp (I am pretty sure you do, anyhow...)
Then everything should play nicely. Let me know if that stream of conscience was unclear :]