Under UNIX, a file opened for appending automatically seeks to the end on every write(). No locking should be required, even with multiple processes appending, as long as every write is accomplished with a single system call (you can check that with strace/truss). If you take a look at the apache source, it doesn't lock the log file when it writes a message, and that's certainly a case where many processes are appending to the same file at once.

In perl, you'll want to set $| on the log file handle, or the output buffers from the different processes will interleave. Or use syswrite.

I don't know if this is true in Windows.