I've found this error too.
It seems to be 99% reproducible in the beta complier. The regular compiler works happily. (Chrome in Ubuntu 10.04)
http://mbed.org/users/giryan/programs/test_workspace_selection/lena0e
If you build that project in the beta compiler then select main.cpp in the workspace frame, then double click on either of the two errors, then the code window changes to the line of the error, but the workspace frame does not change.
I put 99% because I switched back to regular then to beta to check if it was beta only. The very first time I tried it in the beta after being in the regular compiler it correctly changed, but every subsequent time it didn't change.
After double click on an error in the compiler output, you are taken to the problematic line of code - very handy. But the Program Workspace - the lefthand panel in the compiler - does not select the correct file.
This can be frustrating if, for example, you are working on important.c and compile with errors. If you double click on an error to fix it and want to return to important.c you must first select a different file and then select important.c.
It's a minor problem, but it's annoyed me enough times now to report it.